arm: rmobile: Add HopeRun HiHope RZ/G2M board support

The HiHope RZ/G2M board from HopeRun consists of main board
(HopeRun HiHope RZ/G2M main board) and sub board(HopeRun
HiHope RZ/G2M sub board). The HiHope RZ/G2M sub board sits
below the HiHope RZ/G2M main board.

This patch adds the required board support to boot HopeRun HiHope
RZ/G2M board.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>