Make it possible to get/set MMC frequency from camcontrol

[kibab]

Summary

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.

Test Plan

Run camcontrol on a detected card.
Example output for an eMMC card on Rock64:

root@r64:~ # camcontrol mmcsdcmd pass1 -I
Host controller information
Host OCR: 0x300000
Min frequency: 400 KHz
Max frequency: 150 MHz
Supported bus width:
 4 bit
 8 bit
Supported operating modes:
 Can do High Speed transfers
 Can do eMMC HS200 at 1.8V
Supported VCCQ voltages:
 1.8V
 3.3V
Current settings:
 Bus width: 8 bit
 Freq: 150.000 MHz (high-speed timing)
 VCCQ: 1.8V

[manu]

LGTM