dma: mv_xor: do not use pool_size from platform_data within the driver

The driver currently pokes into the platform_data structure during its
normal operation to get the pool_size value. Poking into the
platform_data structure is not nice when moving to the Device Tree, so
this commit adds a new pool_size field in the mv_xor_device structure,
which gets initialized at ->probe() time. The driver then uses this
field instead of the platform_data.

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