mmc: slot-gpio: Add GPIO descriptor based CD GPIO API

Add functions to request a CD GPIO using the GPIO descriptor API.
Note that the new request function is paired with mmc_gpiod_free_cd()
not mmc_gpio_free_cd().  Note also that it must be called prior to
mmc_add_host() otherwise the caller must also call
mmc_gpiod_request_cd_irq().

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>