Control: Make PowerSupplyRedundancy const (read-only)

This property is a policy that dictates how the system would behave
if a power supply goes missing, and it's not intended to be modified
by the user. Set it to read-only.

Tested: Attempting to change the value via REST fails with
"org.freedesktop.DBus.Error.PropertyReadOnly".

Change-Id: I423a49bbf30eb150e38c8da52b913c3097384897
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/xyz/openbmc_project/Control/PowerSupplyRedundancy.interface.yaml b/xyz/openbmc_project/Control/PowerSupplyRedundancy.interface.yaml
index b99d51b..bf36a77 100644
--- a/xyz/openbmc_project/Control/PowerSupplyRedundancy.interface.yaml
+++ b/xyz/openbmc_project/Control/PowerSupplyRedundancy.interface.yaml
@@ -3,5 +3,7 @@
 properties:
     - name: PowerSupplyRedundancyEnabled
       type: boolean
+      flags:
+        - const
       description: >
           When true, the power supplies are redundant.