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: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>