mmc: slot-gpio: add gpiod variant to get wp GPIO

This makes it possible to get the write protect (read only)
GPIO line from a GPIO descriptor. Written to exactly mirror
the card detect function.

Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>