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