Make it possible to get/set MMC frequency from camcontrol

Enhance camcontrol(8) so that it's possible to manually set frequency for SD/MMC cards.
While here, display more information about the current controller, such as
supported operating modes and VCCQ voltages, as well as current VCCQ voltage.

Reviewed by: manu
Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D25795