arm64: dts: imx8mq-librem5: Don't wake up on volume key press

The only key that should wake up the phone is power button press. This
prevents accidental wakeup due to e.g. pressing the buttons in the
pocket or backpack and is in line what userspace uses to unblank the
device.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>