arm64: zynqmp: Add firmware driver

The ZynqMP SoC have a MCU running a firmware to control clocks, resets,
fpga loading etc ...
Add a driver that can be use to communicate with it.
For now only the clock and reset part are implemented.

Differential Revision: https://reviews.freebsd.org/D41811
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit 9e88711f28dc9afa7d68ae8dd027d2399a2a290b)