mmc: pwrseq: Document that simple sequence support more than one GPIO

Many SDIO/MMC attached WLAN chips need more than one ping for their reset
sequence. Extend the pwrseq_simple binding to support more than one pin.

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