amd/amdmct/mct_ddr3: Set prefetch double stride to improve performance

Change-Id: I34ad85388c6b71f0d44bee13afd663e0b84545cd
Signed-off-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Reviewed-on: http://review.coreboot.org/12037
Tested-by: build bot (Jenkins)
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

src/northbridge/amd/amdmct/mct_ddr3/mct_d.c

@@ -5313,6 +5313,7 @@ static void mct_FinalMCT_D(struct MCTStatStruc *pMCTstat,
 				val &= ~(0x7 << 8);		/* CohPrefPrbLmt = 0x1 */
 				val |= (0x1 << 8);
 				val |= (0x1 << 12);		/* EnSplitDctLimits = 0x1 */
+				val |= (0x1 << 20);		/* DblPrefEn = 0x1 */
 				val |= (0x7 << 22);		/* PrefFourConf = 0x7 */
 				val |= (0x7 << 25);		/* PrefFiveConf = 0x7 */
 				val &= ~(0xf << 28);		/* DcqBwThrotWm = 0x0 */