iicbus: Move i2c sensors drivers into new sensor subdirectory

No reason that they should live directly under iicbus

Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D41913

(cherry picked from commit 7c569caa0a6fffa7e1cc0a7f61e986dbc7c59074)