mmccam: Add mmc_sim, a generic sim for mmc driver to use

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