spi: spi-pxa2xx: Add helpers for regiseters' accessing

There are several registers for SPI, and the registers of 'SSCR0' and 'SSCR1'
are accessed frequently. This path is to introduce helper functions to
simplify the accessing of 'SSCR0' and 'SSCR1'.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Weike Chen <alvin.chen@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>