arm64: dts: zynqmp: Add missing lpd watchdog node

Xilinx ZynqMP SoC has FPD (Full Power Domain) and LPD (Low Power Domain)
watchdogs. There are cases where also LPD WDT should be used by Arm cores
that's why list it with disabled status.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/0489a1d5528614f1d570ea153d38b813f0c1eb9f.1611224800.git.michal.simek@xilinx.com