doc: renesas: Document platforms based on RZ/G2 SoC's

Document the platforms based on RZ/G2 SoC's.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Change-Id: I9ce5b9df3573b1198c5c7be79b5471d54573609a
3 files changed