arm64: dts: verdin-imx8mp: unify gpio-key node name

Any other of our modules calls it key-wakeup rather than button-wakeup
so let us stick to that.

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