arm64: zynqmp: Enable cadence WDT for zcu100

Enable watchdog on zcu100 to make sure if there is a bug in the u-boot
there is proper reset.
Watchdog expires and PMU fw is informed and based on setting proper
action is taken.

The patch is enabling reset-on-timeout feature and also fixing fixed
clock rate for watchdog where 100MHz is max (and also default) clock value.

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