clk: Move clock code in dev/clk

We've removed kernel option EXT_RESOURCES almost two years ago.
While it was ok to have some code under a common 'extres' subdirectory
at first, we now have a lot of consumer of it and we made it mandatory
so no need to have it under a cryptic name.

Reviewed by: mhorne
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D43191

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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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 QorIQ platform clockgen driver.

This patch adds classes and functions that can be used with various NXP
QorIQ Layerscape SoCs.

As for the clock topology - there is single platform PLL, which supplies
clocks for the peripheral bus and additional PLLs for CPU cores. There
can be multiple core PLLs (For example - LS1046A has two PLLs - CGAPLL1
and CGAPLL2). Each PLL has fixed dividers on output. The core PLLs
are not accessible from dts.

This is a preparation patch for NXP LS1046A SoC support.

Submitted by: Dawid Gorecki <dgr@semihalf.com>
Reviewed by: mmel
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D24351