dmaengine: mv_xor: add support for a38x command in descriptor mode

The Marvell Armada 38x SoC introduce new features to the XOR engine,
especially the fact that the engine mode (MEMCPY/XOR/PQ/etc) can be part of
the descriptor and not set through the controller registers.

This new feature allows mixing of different commands (even PQ) on the same
channel/chain without the need to stop the engine to reconfigure the engine
mode.

Refactor the driver to be able to use that new feature on the Armada 38x,
while keeping the old behaviour on the older SoCs.

Signed-off-by: Lior Amsalem <alior@marvell.com>
Reviewed-by: Ofer Heifetz <oferh@marvell.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>