dmaengine: rcar-dmac: Implement support for hardware descriptor lists

The DMAC supports hardware-based auto-configuration from descriptor
lists. This reduces the number of interrupts required for processing a
DMA transfer. Support that mode in the driver.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>