mmc: slot-gpio: Record GPIO descriptors instead of GPIO numbers

In preparation for adding a descriptor-based CD GPIO API, switch from
recording GPIO numbers to recording GPIO descriptors.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <chris@printf.net>