Now that mv_xor_device is no longer used to designate the per-channel
DMA devices, use it know to designate the XOR engine themselves
(currently composed of two XOR channels).

So, now we have the nice organization where:

 - mv_xor_device represents each XOR engine in the system
 - mv_xor_chan   represents each XOR channel of a given XOR engine
Signed-off-by: NThomas Petazzoni <thomas.petazzoni@free-electrons.com>