dmaengine: usb-dmac: Add Renesas USB DMA Controller (USB-DMAC) driver
This DMAC is Renesas USB high-speed module DMA controller that
supports slave transfer.
This USB-DMAC has similar register sets with R-Car Gen2 DMAC, but
the USB-DMAC has specific registers to control the USB transactions.
If this code is added into the rcar-dmac driver, it will become
unreadable. So, this driver is independent from the rcar-dmac.