ARM: dts: imx6qdl-dhcom-pdk2: Drop incorrect size-cells in GPIO keys

ARM: dts: imx6qdl-dhcom-pdk2: Drop incorrect size-cells in GPIO keys

Bindings do not allow address/size-cells in GPIO keys and the GPIO keys
is not a bus, see dtbs_check warning:

"
arch/arm/boot/dts/nxp/imx/imx6dl-dhcom-pdk2.dtb: gpio-keys: '#address-cells', '#size-cells' do not match any of the regexes: '^(button|event|key|switch|(button|event|key|switch)-[a-z0-9-]+|[a-z0-9-]+-(button|event|key|switch))$', 'pinctrl-[0-9]+'
"

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>