dt-bindings: dma: renesas,usb-dmac: convert bindings to json-schema

Convert Renesas R-Car USB-DMA Controller bindings documentation
to json-schema.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/1587110829-26609-3-git-send-email-yoshihiro.shimoda.uh@renesas.com


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