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

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.org
Signed-off-by: Rob Herring <robh@kernel.org>