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

Remove a few more stray __FBSDID uses

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D41954

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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 ARMADA38X clkgen driver

This patch introduces clkgen driver for Armada38x SoCs.
Clock topology consists of single coreclk which supplies
clock signal to CPU cores and peripherials.

Reviewed by: manu
Obtained from: Semihalf
Differential Revision: https://reviews.freebsd.org/D36453