mmc: dt: pwrseq-simple: Invent power-off-delay-us

During power off, after the GPIO pin has been asserted, some devices like
the Wifi chip from TI, Wl18xx, needs a delay before the host continues with
clock gating and turning off regulators as to follow a graceful shutdown
sequence.

Therefore invent an optional power-off-delay-us DT binding for
mmc-pwrseq-simple, to allow us to support this constraint.

Cc: devicetree@vger.kernel.org
Cc: Rob Herring <robh+dt@kernel.org>
Cc: linux-mmc@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>

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

@@ -18,6 +18,8 @@ Optional properties:
   "ext_clock" (External clock provided to the card).
 - post-power-on-delay-ms : Delay in ms after powering the card and
 	de-asserting the reset-gpios (if any)
+- power-off-delay-us : Delay in us after asserting the reset-gpios (if any)
+	during power off of the card.
 
 Example: