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: 30af8513 ("dt-bindings: power: add defines for i.MX8MM power domains")
Signed-off-by: NPeng Fan <peng.fan@nxp.com>
Acked-by: NKrzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>

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

@@ -81,6 +81,9 @@ properties:
 
           power-supply: true
 
+          power-domains:
+            maxItems: 1
+
           resets:
             description: |
               A number of phandles to resets that need to be asserted during