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

clk: Move allwinner driver into the common directory

No need to keep it under sys/arm/allwinner/clkng
It's easier to find which controller we support by looking under one directory.
It will also be shared with Allwinner RiscV SoC.

Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D43188