watchdog: xilinx: Read clock frequency directly from DT node

Do not use clock-frequency property from parent node.
Use it from watchdog node.

Signed-off-by: Michal Simek <monstr@monstr.eu>
Acked-By: Alejandro Cabrera <acabrera@udio.cujae.edu.cu>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>