Document support for the Direct Memory Access Controllers (DMAC) in the
Renesas R-Car V4M (R8A779H0) SoC.

Based on a patch in the BSP by Thanh Le.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/96aad3b532ee401f19693e18038494f43ddb90e9.1707915609.git.geert+renesas@glider.beSigned-off-by: Vinod Koul <vkoul@kernel.org>