dmaengine: usb-dmac: Document R8A7799{0,5} bindings

Renesas R-Car D3 (R8A77995) and E3 (R8A77990) SoCs also have the R-Car
gen2/3 compatible DMA controllers, so document the SoC specific binding.

Signed-off-by: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
[uli: squashed]
Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Vinod Koul <vkoul@kernel.org>