allwinner: aw_mmc: Convert driver to use the mmc_sim interface

A lot more generic cam related things are done in mmc_sim so this simplify
the driver a lot.

Differential Revision: https://reviews.freebsd.org/D27486
Reviewed by: imp

(cherry picked from commit 2671bdb540c582e2d44681e4495916629d033e97)