dmaengine: mv_xor: Add support for scatter-gather DMA mode

This patch adds memory to memory scatter-gather support to the Marvell
mv_or DMA driver.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Gregory CLEMENT <gregory.clement@free-electrons.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Marcin Wojtas <mw@semihalf.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>