dma: mv_xor: split initialization/cleanup of XOR channels

Instead of doing the initialization/cleanup of the XOR channels
directly in the ->probe() and ->remove() hooks, we create separate
utility functions mv_xor_channel_add() and mv_xor_channel_remove().

This will allow to easily introduce in a future patch a different way
of registering XOR channels: instead of having one platform_device per
channel, we'll trigger the registration of all XOR channels of a given
XOR engine directly from the XOR engine ->probe() function.

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