watchdog: cadence: Remove driver specific "timeout-sec" handling

Now that we have a generic DT property "timeout-sec" handling, the
driver specific implementation can be dropped.

This patch also changes the timeout restriction to the min and max
values (clipping). Before this patch, the value provided via
"timeout-sec" was used if the parameter was too high or low. Now
the driver specific min and max values are used instead.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Tested-by: Michal Simek <michal.simek@xilinx.com> (on zcu100)