arm64: dts: imx8mq-librem5: add a RO firmware partition

This partition will hold a squashfs firmware jail. Only one read-only
partition is needed.

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