common.Threshold: use dict-of-dict for Value

The Dbus specification does not allow the key of a dict to be a
non-basic type, such as a struct:

    the first single complete type (the "key") must be a basic type
    rather than a container type.

If we try to instantiate an instance with a struct as a dict key, we
end up getting an EINVAL on the add-vtable call.

Switch to nested dictionary to avoid this violation.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I2c512984e62166efec3f299d55efb39a9e93edbf
diff --git a/yaml/xyz/openbmc_project/Common/Threshold.interface.yaml b/yaml/xyz/openbmc_project/Common/Threshold.interface.yaml
index 2186fa8..6e8e673 100644
--- a/yaml/xyz/openbmc_project/Common/Threshold.interface.yaml
+++ b/yaml/xyz/openbmc_project/Common/Threshold.interface.yaml
@@ -5,12 +5,11 @@
 
 properties:
     - name: Value
-      type: dict[struct[enum[self.Type],enum[self.Bound]],double]
+      type: dict[enum[self.Type],dict[enum[self.Bound],double]]
       description: >
           This indicates the threshold ranges applicable for the corresponding
           Value interface. This property is experimental and may change based on
-          implementation.
-          map{struct{thresholdType,thresholdBound},thresholdValue}
+          implementation. map{thresholdType,map{thresholdBound,thresholdValue}}
       flags:
           - readonly
     - name: Asserted