dma: mv_xor: allow channels to be registered directly from the main device

Extend the XOR engine driver (currently called "mv_xor_shared") so
that XOR channels can be passed in the platform_data structure, and be
registered from there.

This will allow the users of the driver to be converted to the single
platform_driver variant of the mv_xor driver.

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