dmaengine: rcar-dmac: Document R8A774A1 bindings

Renesas' RZ/G2M (R8A774A1) SoC has DMA controllers compatible
with this driver, therefore document RZ/G2M specific bindings.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Vinod Koul <vkoul@kernel.org>