ARM: dts: imx6ull-colibri: move gpio-keys node to som dtsi

The gpio-keys define module level wake-up pin functionality. Move it
from the carrier board dts file to the Som dtsi file.
While at it, also re-order the properties in the gpio-keys node
alphabetically and rename to sub-node from power to wakeup.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>