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: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>