arm64: dts: imx8mm-venice-gw790: Pass GSC address/size-cells

arm64: dts: imx8mm-venice-gw790: Pass GSC address/size-cells

Per gateworks-gsc.yaml, #address-cells and #size-cells are mandatory
properties.

Pass them to fix the following schema warning:

imx8mm-venice-gw7903.dtb: gsc@20: '#address-cells' is a required property
	from schema $id: http://devicetree.org/schemas/mfd/gateworks-gsc.yaml#
imx8mm-venice-gw7903.dtb: gsc@20: '#size-cells' is a required property
	from schema $id: http://devicetree.org/schemas/mfd/gateworks-gsc.yaml#

Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>