spi/xilinx: merge OF support code into main driver

Now that the of_platform_bus_type has been merged with the platform
bus type, a single platform driver can handle both OF and non-OF use
cases.  This patch merges the OF support into the platform driver.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Tested-by: Michal Simek <monstr@monstr.eu>