mfd: Allow the platform to specify the sh_mobile_sdhi get_cd handler

On some platforms (e.g. AP4EVB) the card detect pin of a slot is not
directly connected to the sdhi hardware, so that polling needs to be used
with tmio_mmc and card detection is handled in the platform code.
This patch allows to set tmio_mmc capabilities (to pass the
MMC_CAP_NEEDS_POLL flag) and 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>