ARM: dts: imx6qdl: mba6: Fix gpio-keys button node names

Numbers are separated by dashes. Fixes the warnings:
arch/arm/boot/dts/nxp/imx/imx6q-mba6a.dtb: gpio-buttons: 'button1',
 'button2', 'button3' 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]+'
 from schema $id: http://devicetree.org/schemas/input/gpio-keys.yaml#Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/nxp/imx/imx6qdl-mba6.dtsi

@@ -36,21 +36,21 @@ gpio_buttons: gpio-buttons {
 		pinctrl-names = "default";
 		pinctrl-0 = <&pinctrl_gpiobuttons>;
 
-		button1 {
+		button-1 {
 			label = "s6";
 			linux,code = <KEY_F6>;
 			gpios = <&gpio7 13 GPIO_ACTIVE_LOW>;
 			wakeup-source;
 		};
 
-		button2 {
+		button-2 {
 			label = "s7";
 			linux,code = <KEY_F7>;
 			gpios = <&gpio7 12 GPIO_ACTIVE_LOW>;
 			wakeup-source;
 		};
 
-		button3 {
+		button-3 {
 			label = "s8";
 			linux,code = <KEY_F8>;
 			gpios = <&gpio1 8 GPIO_ACTIVE_LOW>;