Implement initial MMC partitions support for MMCCAM.

For MMC cards, add partitions found on the card as separate disk(9) devices.
Don't do anything with RPMB partition for now.
Lots of code is copied almost 1:1 from the mmcsd.c in the old stack,
credits Marius Strobl (marius@FreeBSD.org)

Reviewed by: marius
Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D12762