arm64: dts: renesas: beacon-rzg2m-kit: Rearrange SoC unique functions

In preparation for adding new dev kits, move anything specific to the
RZ/G2M from the SOM-level and baseboard-levels and move them to the
kit-level.  This allows the SOM and baseboard to be reused with
other SoC's.

Signed-off-by: Adam Ford <aford173@gmail.com>
Link: https://lore.kernel.org/r/20201224170502.2254683-6-aford173@gmail.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>