dt-bindings: dma: Document RZ/G2L bindings

Document RZ/G2L DMAC bindings.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210806095322.2326-2-biju.das.jz@bp.renesas.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>