dt-bindings: power: fsl,imx-gpc: document interrupt-controller

The fsl,imx6q-gpc is used as interrupt controller (routing back to GIC):

  imx6sl-evk.dtb: gpc@20dc000: '#interrupt-cells', 'interrupt-controller' do not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>