arm64: zynqmp: Add clock driver

[manu]

Summary

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.

Sponsored by: Beckhoff Automation GmbH & Co. KG

[manu]

Never register external clock if they are not in the clock-names property.
Register the dummy clock as a fixed clock with a frequency of 0. It's used
to set as parent for clock that have non-resolved external clock.