mmc: Allow the tmio_mmc mfd driver to specify get_cd handler

Some controllers, supported by the tmio_mmc driver do not have the card
detect pin of a slot connected, so that polling needs to be used and
card detection is handled by other means.
This patch exposes a get_cd hook for that purpose.

Signed-off-by: Arnd Hannemann <arnd@arndnet.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>