arm64: dts: renesas: Add HiHope RZ/G2M sub board support

The HiHope RZ/G2M sub board sits below the HiHope RZ/G2M main board.
This patch also adds ethernet support along with a dtsi common to
both HiHope RZ/G2M and RZ/G2N sub boards.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>