dmaengine: mv_xor: improve descriptors list handling and reduce locking

This patch change the way free descriptors are marked.

Instead of having a field for descriptor in use, all the descriptors in the
all_slots list are free for use.

This simplify the allocation method and reduce the locking needed.

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>