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: [e-mail]
Cc: Rob Herring <[e-mail]>
Cc: [e-mail]
Signed-off-by: Ulf Hansson <[e-mail]>
Acked-by: Arnd Bergmann <[e-mail]>

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

···
   "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: