dt-bindings: rcar-dmac: Document r8a77470 support

Renesas  RZ/G SoC also have the R-Car gen2/3 compatible DMA controllers.
Document RZ/G1C (also known as R8A77470) SoC bindings.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>