dmaengine: rcar-dmac: Document R8A774C0 bindings

Renesas' RZ/G2E (R8A774C0) SoC has DMA controllers compatible
with this driver, therefore document RZ/G2E specific bindings.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Vinod Koul <vkoul@kernel.org>