Documentation: mmc: add mmc-hs400-enhanced-strobe

mmc-hs400-enhanced-strobe is used to claim that the
host can support hs400 mode with enhanced strobe
introduced by emmc 5.1 spec.

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

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>