Add MMCCAM support to AllWinner MMC driver

Using MMCCAM on AllWinner boards is now possible, reaching highest
possible data transfer speed.

For now, MMCCAM doesn't scan cards on boot. This means that scanning
has to be done manually and that it's not possible to mount root FS
from MMC/SD card since there is no block device at the boot time.

For manually scanning the cards, run:

camcontrol rescan X:0:0

Where X is the bus number (look at camcontrol devlist to determine
bus number assigned to the MMC controller).

Reviewed by: manu
Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D15891