fsl-ddr: add override for the Rtt_Wr

Different boards may require different settings of Dynamic ODT (Rtt_Wr).
We provide a means to allow the board specific code to provide its own
value of Rtt_Wr.

Signed-off-by: Dave Liu <daveliu@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
diff --git a/include/asm-ppc/fsl_ddr_sdram.h b/include/asm-ppc/fsl_ddr_sdram.h
index 15ab675..3216a50 100644
--- a/include/asm-ppc/fsl_ddr_sdram.h
+++ b/include/asm-ppc/fsl_ddr_sdram.h
@@ -192,6 +192,7 @@
 	/* Rtt impedance */
 	unsigned int rtt_override;		/* rtt_override enable */
 	unsigned int rtt_override_value;	/* that is Rtt_Nom for DDR3 */
+	unsigned int rtt_wr_override_value;	/* this is Rtt_WR for DDR3 */
 
 	/* Automatic self refresh */
 	unsigned int auto_self_refresh_en;