ARM: dts: imx6ul: mba6ulx: Fix gpio-keys button node names

ARM: dts: imx6ul: mba6ulx: Fix gpio-keys button node names

Numbers are separated by dashes. Fixes the warnings:
arch/arm/boot/dts/nxp/imx/imx6ull-tqma6ull2-mba6ulx.dtb: gpio-keys:
'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>