arm: dove: Convert Dove to DT XOR DMA engine

With DT support for Marvell XOR DMA engine, make use of it on Dove.
Also remove the now redundant code in DT board init for xor engines.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>