dmaengine: rcar-dmac: Document R8A77990 bindings

Renesas R-Car E3 (R8A77990) SoC also has the R-Car gen2/3 compatible DMA
controllers, so document the SoC specific binding.

Signed-off-by: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/dma/renesas,rcar-dmac.txt

@@ -29,6 +29,7 @@ Required Properties:
 		- "renesas,dmac-r8a77965" (R-Car M3-N)
 		- "renesas,dmac-r8a77970" (R-Car V3M)
 		- "renesas,dmac-r8a77980" (R-Car V3H)
+		- "renesas,dmac-r8a77990" (R-Car E3)
 		- "renesas,dmac-r8a77995" (R-Car D3)
 
 - reg: base address and length of the registers block for the DMAC