spi: pxa2xx: Cleanup register access macros

Currently SSP registers are accessed by having an own read and write macros
for each register. For instance read_SSSR(iobase) and write_SSSR(iobase).

In my opinion this hurts readability and requires new macros to be defined
for each new added register. Let's define and use instead common
pxa2xx_spi_read() and pxa2xx_spi_write() accessors.

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