mmc: tegra: Implement HS400 enhanced strobe

Implement eMMC HS400 enhanced strobe. Enhanced strobe is an alternative
mechanism to the HS400 tuning procedure.

Signed-off-by: Aapo Vienamo <avienamo@nvidia.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>