mmc: slot-gpio: add a helper to check capability of GPIO WP detection

Like mmc_can_gpio_cd(), mmc_can_gpio_ro() will also be useful for host
drivers to know whether GPIO write-protect detection is supported.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>