xilinx_spi: add a platform driver using the xilinx_spi common module.

This patch adds in a platform device driver using the xilinx_spi common module.

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>