spi/xilinx: Remove remains of of_platform device registration

In the past there used to be a separate platform device type for device
tree systems so the probe and removal functions were split into generic
and bus sections. Since this is no longer the case simplify the code (and
remove some unprototyped exports) by factoring everything into the bus
probe() and remove().

Signed-off-by: Mark Brown <broonie@linaro.org>