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 <[e-mail]>
Reviewed-by: Oleksij Rempel <[e-mail]>
Signed-off-by: Shawn Guo <[e-mail]>

arch/arm/boot/dts/imx6dl-plybas.dts

···
 
 	gpio_keys {
 		compatible = "gpio-keys";
-		#address-cells = <1>;
-		#size-cells = <0>;
 		autorepeat;
 
-		button@20 {
+		button-start {
 			label = "START";
 			linux,code = <31>;
 			gpios = <&gpio5 8 GPIO_ACTIVE_LOW>;
 		};
 
-		button@21 {
+		button-clean {
 			label = "CLEAN";
 			linux,code = <46>;
 			gpios = <&gpio5 9 GPIO_ACTIVE_LOW>;