Documentation: mmc: addtional description for post-power-on-delay-ms

post-power-on-delay-ms woule be reused to substitute the hard-coded 10ms
delay waiting for power supply to be stable, specificed by individual
platform/board. Default to 10ms as before, if no available.
Signed-off-by: NShawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/mmc.txt

@@ -58,6 +58,10 @@ Optional properties:
 - fixed-emmc-driver-type: for non-removable eMMC, enforce this driver type.
   The value <n> is the driver type as specified in the eMMC specification
   (table 206 in spec version 5.1).
+- post-power-on-delay-ms : It was invented for MMC pwrseq-simple which could
+  be referred to mmc-pwrseq-simple.txt. But now it's reused as a tunable delay
+  waiting for I/O signalling and card power supply to be stable, regardless of
+  whether pwrseq-simple is used. Default to 10ms if no available.
 
 *NOTE* on CD and WP polarity. To use common for all SD/MMC host controllers line
 polarity properties, we have to fix the meaning of the "normal" and "inverted"