mmc: pwrseq_simple: Add support for a reset GPIO pin

The need for reset GPIOs has several times been pointed out from
erlier posted patchsets. Especially some WLAN chips which are
attached to an SDIO interface may use a GPIO reset.

The reset GPIO is asserted at initialization and prior we start the
power up procedure. The GPIO will be de-asserted right after the power
has been provided to the card, from the ->post_power_on() callback.

Note, the reset GPIO is optional. Thus we don't return an error even if
we can't find a GPIO for the consumer.

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