mmc: Allow the platform to specify the sh_mmcif get_cd handler

In some platforms (e.g. AP4EVB) the card detect pin of a slot is not
directly connected to the sh_mmcif controller, so that polling needs
to be used. To overcome the overhead induced by querying the controller
on each poll cycle, card detection can be handled in the platform code
more efficiently.
This patch exposes a get_cd hook for that purpose.

Signed-off-by: Arnd Hannemann <arnd@arndnet.de>
Tested-by: Yusuke Goda <yusuke.goda.sx@renesas.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>