mmc: sh_mobile_sdhi: support modular mmc-core with non-standard hotplug

Currently if a platform wants to implement a non-standard card-detection
method, it would need to call tmio_mmc_cd_wakeup(), which is an inline
function, calling mmc_detect_change(). For this the platform would have
to link mmc_core statically into the kernel, losing the ability to build
it as a module. This patch adds a callback to the sh_mobile_sdhi driver,
which eliminates this dependency.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Chris Ball <cjb@laptop.org>