dma: mv_xor: change the driver name to 'mv_xor'

Since we got rid of the per-XOR channel 'mv_xor' driver, now the
per-XOR engine driver that used to be called 'mv_xor_shared' can
simply be named 'mv_xor'.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>