emmc: add macros CMD21, BUS_WIDTH_DDR_4 and BUS_WIDTH_DDR_8

Add some macros according to JEDEC Standard Embedded Multi-Media
Card (eMMC) Electrical Standard (5.1)": Table 145 - Bus Mode
Selection.

Change-Id: Iaa45e0582653ef4290efd60d039f0bdc420eeb47
Signed-off-by: Qixiang Xu <qixiang.xu@arm.com>
diff --git a/drivers/emmc/emmc.c b/drivers/emmc/emmc.c
index bcdc82c..92d1e87 100644
--- a/drivers/emmc/emmc.c
+++ b/drivers/emmc/emmc.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016-2017, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2016-2018, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  *
@@ -353,7 +353,9 @@
 	       (clk != 0) &&
 	       ((width == EMMC_BUS_WIDTH_1) ||
 		(width == EMMC_BUS_WIDTH_4) ||
-		(width == EMMC_BUS_WIDTH_8)));
+		(width == EMMC_BUS_WIDTH_8) ||
+		(width == EMMC_BUS_WIDTH_DDR_4) ||
+		(width == EMMC_BUS_WIDTH_DDR_8)));
 	ops = ops_ptr;
 	emmc_flags = flags;