ARM: dts: imx7s: Remove #power-domain-cells from gpcv2

ARM: dts: imx7s: Remove #power-domain-cells from gpcv2

Per fsl,imx-gpcv2.yaml, '#power-domain-cells' is not a valid
property for the top-level gpcv2 node. It is only valid
for its children nodes.

Remove it to fix the following schema warning:

gpc@303a0000: '#power-domain-cells' does not match any of the regexes: 'pinctrl-[0-9]+'
from schema $id: http://devicetree.org/schemas/power/fsl,imx-gpcv2.yaml#

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>