dmaengine: renesas,usb-dmac: Add device tree bindings documentation

Document the device tree bindings for the Renesas USB DMA
Controller (USB-DMAC).

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>