Butterfly: Force DDR refresh rate to 2x.

Due to OEM request, always set DDR refresh rate at 2x.

Change-Id: I81a4f57aca6388551dca6effbd9a4ac1a97e4f5a
Reviewed-on: https://gerrit.chromium.org/gerrit/50477
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Tested-by: Shawn Nematbakhsh <shawnn@chromium.org>
Commit-Queue: Shawn Nematbakhsh <shawnn@chromium.org>
Reviewed-on: http://review.coreboot.org/4214
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/mainboard/google/butterfly/romstage.c

@@ -162,6 +162,7 @@ void main(unsigned long bist)
 			{ 0, 4, 0x0000 }, /* P12: Empty */
 			{ 0, 4, 0x0000 }, /* P13: Empty */
 		},
+		ddr_refresh_rate_config: 2, /* Force double refresh rate */
 	};
 
 	timestamp_init(get_initial_timestamp());