spi: pxa2xx: Add definition for Intel Quark DDS_RATE register

Intel Quark DDS_RATE register is defined only in register access macro. Add
a definition for it to common SSP register definitions for preparing to
cleanup those macros.

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