Document RZ/G2M (R8A774A1) SoC bindings.

The RZ/G2M SoC is similar to R-Car M3-W (R8A7796).
Signed-off-by: NBiju Das <biju.das@bp.renesas.com>
Reviewed-by: NFabrizio Castro <fabrizio.castro@bp.renesas.com>
Acked-by: NNiklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: NSimon Horman <horms+renesas@verge.net.au>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NHans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>