arm64: dts: split out a shared imx8mq-librem5-r3.dtsi description

The Librem 5 r3 ("Dogwood") and r4 ("Evergreen") revisions are quite
similar. Add a shared imx8mq-librem5-r3.dtsi description to be included
in r3 and later dts files in order to avoid duplication.

This is no change in the descriptions but only refactoring.

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