watchdog: xilinx: Use of_property_read_u32

Use of_property_read_u32 functions to clean probe function.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>