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

arm64: zynqmp: Add clock driver

Add clock and reset drivers for the ZynqMP SoC.
The clocks are discovered by talking to the firmware as the topology isn't
fixed on this SoC.

Differential Revision: https://reviews.freebsd.org/D41812
Sponsored by: Beckhoff Automation GmbH & Co. KG