arm64: dts: imx8mq-librem5: add the magnetometer mount matrix

Userland needs the mount matrix to know the correct orientation of
the part.

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>