dt-bindings: arm/cpus: Add power-domains constraints

The "power-domains" and "power-domains-names" properties are missing any
constraints. Add the constraints and drop the generic descriptions.

Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Reviewed-by: Ulf Hansson <[e-mail]>
Reviewed-by: Sudeep Holla <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring (Arm) <[e-mail]>

Documentation/devicetree/bindings/arm/cpus.yaml

···
     maxItems: 1
 
   power-domains:
-    description:
-      List of phandles and PM domain specifiers, as defined by bindings of the
-      PM domain provider (see also ../power_domain.txt).
+    maxItems: 1
 
   power-domain-names:
     description:
-      A list of power domain name strings sorted in the same order as the
-      power-domains property.
-
       For PSCI based platforms, the name corresponding to the index of the PSCI
       PM domain provider, must be "psci". For SCMI based platforms, the name
       corresponding to the index of an SCMI performance domain provider, must be
       "perf".
+    enum: [ psci, perf, cpr ]
 
   resets:
     maxItems: 1