spi: pxa2xx-pci: Do a specific setup in a separate function

Move LPSS specific setup to a separate function. It makes ->probe() cleaner as
well as allows extend the driver for different variation of hardware in the
future, e.g. for Intel Merrifield.

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