dt-bindings: power: gpcv2: add power-domains property

Some pgc power-domain requires a parent power domain, so
add an optional power-domains property, otherwise there will be
dt check warning:
gpc@303a0000: pgc:power-domain@1: 'power-domains' does not match
any of the regexes: 'pinctrl-[0-9]+'

Fixes: 30af8513bdb5 ("dt-bindings: power: add defines for i.MX8MM power domains")
Signed-off-by: Peng Fan <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Shawn Guo <[e-mail]>

Documentation/devicetree/bindings/power/fsl,imx-gpcv2.yaml

···
 
           power-supply: true
 
+          power-domains:
+            maxItems: 1
+
           resets:
             description: |
               A number of phandles to resets that need to be asserted during