ARM: dts: imx7-colibri: add GPIO wakeup key

Add wakeup GPIO key which is able to wake the system from sleep
modes (e.g. Suspend-to-Memory).

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>