mmc: sdhci-xenon: Support HS400 Enhanced Strobe feature

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: Hu Ziji <huziji@marvell.com>
Signed-off-by: Zhoujie Wu <zjwu@marvell.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>