dt-bindings: power: fsl,imx-gpcv2: Document interrupt controller properties

The i.MX General Power Controller v2 is also an interrupt controller so
document additional properties to fix dtbs_check warnings like:

  arch/arm64/boot/dts/freescale/imx8mq-evk.dt.yaml: gpc@303a0000:
    '#interrupt-cells', 'interrupt-controller' do not match any of the regexes: 'pinctrl-[0-9]+'
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200904145312.10960-2-krzk@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

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

@@ -33,6 +33,10 @@ properties:
   interrupts:
     maxItems: 1
 
+  interrupt-controller: true
+  '#interrupt-cells':
+    const: 3
+
   pgc:
     type: object
     description: list of power domains provided by this controller.