xilinx_spi: Split into of driver and generic part.

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: John Linn <John.Linn@xilinx.com>
Signed-off-by: Richard Röjfors <richard.rojfors@mocean-labs.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>