mmc: sdhci-of-arasan: implement enhanced strobe callback

Currently sdhci-arasan 5.1 can support enhanced strobe function,
and we now limit it just for "arasan,sdhci-5.1". Add
mmc-hs400-enhanced-strobe in DT to enable the function if we're
sure our controller can support it.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>

Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Tested-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>