spi: pxa2xx: Drop linux/spi/pxa2xx_spi.h

Merge series from Andy Shevchenko <andriy.shevchenko@linux.intel.com>:

As Arnd suggested we may drop linux/spi/pxa2xx_spi.h as most of
its content is being used solely internally to SPI subsystem
(PXA2xx drivers). Hence this refactoring series with the additional
win of getting rid of legacy documentation.

Note, that we have the only user of a single plain integer field
in the entire kernel for that. Switching to software nodes does not
diminish any of type checking as we only pass an integer.