watchdog: Add Cadence watchdog driver

This IP can be found on Zynq and ZynqMP devices.
The driver was tested with reset-on-timeout; feature.

Also adding WATCHDOG symbol to Kconfig because it is required.

Signed-off-by: Shreenidhi Shedi <imshedi@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>