Add support for the Allwinner i2c device. This is similar to the existing
Marvell twsi part, however uses different register locations, as such split
the existing driver into Marvell and Allwinner attachments.

While here clean a few style issues.

Submitted by: Emmanuel Vadot <manu@bidouilliste.com>
Differential Revision: https://reviews.freebsd.org/D4846