This patch splits the xilinx_spi driver into a generic part and a
OF driver part.

The reason for this is to later add in a platform driver as well.
Tested-by: NJohn Linn <John.Linn@xilinx.com>
Signed-off-by: NRichard Röjfors <richard.rojfors@mocean-labs.com>
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>