ARM: dts: imx6qdl: vicut1: rename power to power-button

ARM: dts: imx6qdl: vicut1: rename power to power-button

In order to address the warnings about the naming convention for keys,
this commit renames the 'power' key to 'power-button' in the vicut1-12inch
device tree source file. The warning in question is as follows:
arch/arm/boot/dts/imx6q-vicut1.dtb: gpio-keys: 'power' does 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]+'

The referenced warning originates from the following schema:
Documentation/devicetree/bindings/input/gpio-keys.yaml

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>