Support HS400 Enhanced Strobe feature in Xenon.

Enable Enhanced Strobe together with Data Strobe.
Disable Enhanced Strobe when eMMC is not in HS400 mode.
Signed-off-by: NHu Ziji <huziji@marvell.com>
Signed-off-by: NZhoujie Wu <zjwu@marvell.com>
Acked-by: NAdrian Hunter <adrian.hunter@intel.com>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>