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

Basic support for the HiHope RZ/G2M main board:
  - Memory,
  - Main crystal,
  - Serial console

This patch also includes a dtsi common to both HiHope RZ/G2M
and RZ/G2N main 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>