sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

Add a skeleton Clock Manager for RPi2/3, and use that from pwm
instead of frobbing the registers directly.

As a hack the bcm2835_pwm kmod presently ignores the 'status="disabled"'
in the RPI3 DTB, assuming that if you load the kld you probably
want the PWM to work.