ARM: zynq: DT: Add 'clock-latency' property

Specify the 'clock-latency' property to avoid certain cpufreq governors
from refusing to work with the following error:
  ondemand governor failed, too long transition latency of HW, fallback to performance governor

Reported-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Tested-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>