arm64: dts: imx8mq-librem5: make the volume buttons a wakeup source

They turn on the display currently so to be consistent, let's make
them wake the system from suspend as well.

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>