mmc: slot-gpio: Add a function to enable/disable card detect IRQ wakeup

Commit 03dbaa04 ("mmc: slot-gpio: Add support to enable irq wake on
cd_irq") enabled wakeup at initialization. However drivers may wish to
enable and disable based on different criteria. Add a helper function
mmc_gpio_set_cd_wake() to make it easy for drivers to do that.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>