arm64: zynqmp: Disable WDT for zcu100

Do not enable WDT by default on this target because distributions are
not enabling watchdog driver to service it.

Feature has been enabled by:
"arm64: zynqmp: Enable cadence WDT for zcu100"
(sha1: 767afebb)

And WDT is still enabled in rebranded Avnet Ultra 96 board support.

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