dma: mv_xor: rename mv_xor_platform_data to mv_xor_channel_data

mv_xor_platform_data used to be the platform_data structure associated
to the 'mv_xor' driver. This driver no longer exists, and this data
structure really contains the properties of each XOR channel part of a
given XOR engine. Therefore 'struct mv_xor_channel_data' is a more
appropriate name.

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