iicbus: Move twsi under a new controller subdirectory

The folder is a mess so start moving stuff into sub-directories.

Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D41907

(cherry picked from commit 580d00f42fdd94ce43583cc45fe3f1d9fdff47d4)