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>
2 files changed