iicbus: Move adm1030 and adt746x to new pwm subdirectory

Those are (mainly) pwm controller so move it under a new subdirectory.

Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D41910

(cherry picked from commit 22d7dd834bc5cd189810e414701e3ad1e98102e4)