powerpc/mpc8xxx: Allow override DDR read-to-write turnaround time

Add this option to allow boards to override the default read-to-write
turnaround time for better performance.

Signed-off-by: York Sun <yorksun@freescale.com>

arch/powerpc/include/asm/fsl_ddr_sdram.h

@@ -271,6 +271,9 @@ typedef struct memctl_options_s {
 	unsigned int rcw_2;
 	/* control register 1 */
 	unsigned int ddr_cdr1;
+
+	unsigned int trwt_override;
+	unsigned int trwt;			/* read-to-write turnaround */
 } memctl_options_t;
 
 extern phys_size_t fsl_ddr_sdram(void);