mmc: dw_mmc: use slot-gpio to handle cd pin

Suggested by Jaehoon: Use slot-gpio to handle cd-gpio
Add function dw_mci_of_get_cd_gpio to check "cd-gpios" from dts.
mmc_gpio_request_cd and mmc_gpio_get_cd are used to handle cd pin

Signed-off-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Chris Ball <chris@printf.net>