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>

include/linux/pxa2xx_ssp.h

@@ -37,6 +37,7 @@
 #define SSDR		(0x10)  /* SSP Data Write/Data Read Register */
 
 #define SSTO		(0x28)  /* SSP Time Out Register */
+#define DDS_RATE	(0x28)  /* SSP DDS Clock Rate Register (Intel Quark) */
 #define SSPSP		(0x2C)  /* SSP Programmable Serial Protocol */
 #define SSTSA		(0x30)  /* SSP Tx Timeslot Active */
 #define SSRSA		(0x34)  /* SSP Rx Timeslot Active */