dma: mv_xor: in mv_xor_device, rename 'common' to 'dmadev'

The mv_xor_device structure embeds a 'struct dma_device', which is
named 'common', a not very meaningful name. Rename it to 'dmadev',
which will help avoid confusions later as we merge the mv_xor_device
and mv_xor_chan structures together.

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