mmc: core: expose the capability of gpio card detect

Add new helper API mmc_can_gpio_cd for slot-gpio to make
host drivers know whether it supports gpio card detect.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>