dmaengine: rcar-dmac: group slave configuration

Group slave address and transfer size in own structs for source and
destination. This is in preparation for hooking up the dma-mapping API
to the slave addresses.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>