ARM: dts: imx6dl-plybas: Fix gpio-keys W=1 warnings

Remove the unnecessary #address-cells/#size-cells and rename the node
names to fix the following W=1 dtc warnings:

arch/arm/boot/dts/imx6dl-plybas.dts:26.13-30.5: Warning (unit_address_vs_reg): /gpio_keys/button@20: node has a unit name, but no reg or ranges property
arch/arm/boot/dts/imx6dl-plybas.dts:32.13-36.5: Warning (unit_address_vs_reg): /gpio_keys/button@21: node has a unit name, but no reg or ranges property
arch/arm/boot/dts/imx6dl-plybas.dts:20.12-37.4: Warning (avoid_unnecessary_addr_size): /gpio_keys: unnecessary #address-cells/#size-cells without "ranges" or child "reg" property

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>