mmc: pwrseq: Document optional clock for the simple power sequence

Some WLAN chips attached to a SDIO interface, need an external clock
to be operational. Since this is very common, extend the simple MMC
power sequence DT binding to support an optional clock.
Signed-off-by: NJavier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/mmc-pwrseq-simple.txt

@@ -12,6 +12,10 @@ Optional properties:
 	at initialization and prior we start the power up procedure of the card.
 	They will be de-asserted right after the power has been provided to the
 	card.
+- clocks : Must contain an entry for the entry in clock-names.
+  See ../clocks/clock-bindings.txt for details.
+- clock-names : Must include the following entry:
+  "ext_clock" (External clock provided to the card).
 
 Example: