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>

Documentation/devicetree/bindings/mmc/mmc.txt

@@ -46,6 +46,7 @@ Optional properties:
 - mmc-hs200-1_2v: eMMC HS200 mode(1.2V I/O) is supported
 - mmc-hs400-1_8v: eMMC HS400 mode(1.8V I/O) is supported
 - mmc-hs400-1_2v: eMMC HS400 mode(1.2V I/O) is supported
+- mmc-hs400-enhanced-strobe: eMMC HS400 enhanced strobe mode is supported
 - dsr: Value the card's (optional) Driver Stage Register (DSR) should be
   programmed with. Valid range: [0 .. 0xffff].