Control.Power.Mode: Add AllowedPowerModes property

Add new property that defines what Power Modes are supported.

Tested on Rainier.

'''
busctl -l introspect org.open_power.OCC.Control /xyz/openbmc_project/control/host0/power_mode
NAME                                   TYPE      SIGNATURE RESULT/VALUE                                                                                                                                                                                                                                                                                                                                                                                                                  FLAGS
...
xyz.openbmc_project.Control.Power.Mode interface -         -                                                                                                                                                                                                                                                                                                                                                                                                                             -
.AllowedPowerModes                     property  as        3 "xyz.openbmc_project.Control.Power.Mode.PowerMode.Static" "xyz.openbmc_project.Control.Power.Mode.PowerMode.MaximumPerformance" "xyz.openbmc_project.Control.Power.Mode.PowerMode.PowerSaving" const
.PowerMode                             property  s         "xyz.openbmc_project.Control.Power.Mode.PowerMode.MaximumPerformance"                                                                                                                                                                                                                                                                                                                                                         emits-change writable
.SafeMode                              property  b         false                                                                                                                                                                                                                                                                                                                                                                                                                         emits-change
'''

Change-Id: I4b8285adaabb2aa42592e73f259d9636832f904b
Signed-off-by: Chris Cain <cjcain@us.ibm.com>
diff --git a/yaml/xyz/openbmc_project/Control/Power/Mode.interface.yaml b/yaml/xyz/openbmc_project/Control/Power/Mode.interface.yaml
index 1050255..54fe794 100644
--- a/yaml/xyz/openbmc_project/Control/Power/Mode.interface.yaml
+++ b/yaml/xyz/openbmc_project/Control/Power/Mode.interface.yaml
@@ -4,10 +4,12 @@
 properties:
     - name: PowerMode
       type: enum[self.PowerMode]
+      default: Static
       description: >
           This property shall contain the computer system power mode setting.
           This defines the processor speed based on the priority of power
           consumption and performance.
+
     - name: SafeMode
       type: boolean
       flags:
@@ -17,6 +19,16 @@
           This property shall indicate whether the System is in Safe Mode. When
           this is true, the system power mode is not being used in the system.
 
+    - name: AllowedPowerModes
+      type: set[enum[self.PowerMode]]
+      flags:
+          - const
+      description: >
+          A const property describing all the allowed power modes. Some systems
+          may not support all modes, so this property can be filled in with only
+          the supported modes. If empty, then any of the PowerMode values can be
+          set.
+
 enumerations:
     - name: PowerMode
       description: >