dma: mv_xor: remove the pool_size from platform_data

The pool_size is always PAGE_SIZE, and since it is a software
configuration paramter (and not a hardware description parameter), we
cannot make it part of the Device Tree binding, so we'd better remove
it from the platform_data as well.

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