This adds a generic sim that abstract a lot of what needs to be implemented
in a driver for mmccam support.
A new interface with three methods is added :
- mmc_sim_get_tran_settings: Use to get what the controller supports in term
of capabilities, freq etc ...
- mmc_sim_set_tran_settings: Use to change the speed/freq/etc of the
sdcard host controller
- mmc_sim_cam_request: Used for MMCIO requests
Differential Revision: https://reviews.freebsd.org/D27485
Reviewed by: kibab
