iicbus: Move remaining rtc driver into rtc subfolder

No reason that they should live directly under iicbus

Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D41908

sys: Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Build the ds1672 driver as a module. Add a detach() to unregister the rtc.