arm64: zynqmp: Use assigned-clock-rates for setting up clock in SOM

arm64: zynqmp: Use assigned-clock-rates for setting up clock in SOM

With limited low level configuration done via psu-init only IPs connected
on SOM are initialized and configured. All IPs connected to carrier card
are not initialized. There is a need to do proper reset, pin configuration
and also clock setting.
The patch targets the last part which is setting up proper clock for USBs
and SDs which on kr260 and kv260-revB boards.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>