SD : Updated CMD 6 implememtation.

For setting high speed in SD card,
First CMD 6 (Switch) is send to check if card supports High Speed and
Second command is send to switch card to high speed mode.

In current inplementation, CMD 6 was sent only once to switch the
card into HS mode without checking if card supports HS or not, which is
not as per specification and also we are not setting the HS i.e. 50000000
but directly asking the card to switch to 26000000 which is incorrect as
SD card supports either 25000000 or 50000000.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Meenakshi Aggarwal <meenakshi.aggarwal@nxp.com>
Reviewed-by: Jun Nie <jun.nie@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>

EmbeddedPkg/Universal/MmcDxe/Mmc.h

@@ -64,6 +64,14 @@
 #define EMMC_CMD6_ARG_VALUE(x)              (((x) & 0xFF) << 8)
 #define EMMC_CMD6_ARG_CMD_SET(x)            (((x) & 0x7) << 0)
 
+#define SWITCH_CMD_DATA_LENGTH              64
+#define SD_HIGH_SPEED_SUPPORTED             0x20000
+#define SD_DEFAULT_SPEED                    25000000
+#define SD_HIGH_SPEED                       50000000
+#define SWITCH_CMD_SUCCESS_MASK             0x0f000000
+
+#define BUSWIDTH_4                          4
+
 typedef enum {
   UNKNOWN_CARD,
   MMC_CARD,              //MMC card