spi: pxa2xx-pci: Drop unused header inclusion

There is nothing in the driver which requires OF specific header
to be included.

Remove it for good.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>