ARM: dts: imx6q-var-dt6customboard: Remove #address/#size-cells from gpio-keys

Remove unnecessary #address-cells/#size-cells to fix the following
DTC warning:

arch/arm/boot/dts/imx6q-var-dt6customboard.dtb: Warning (avoid_unnecessary_addr_size): /gpio-keys: unnecessary #address-cells/#size-cells without "ranges" or child "reg" property

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>