Documentation/watchdog: add timeout and ping rate control to watchdog-test.c

The watchdog test program is much more useful if it can configure the
timeout value and ping rate.  This will allow you to test actual timeouts.

Adds the -t parameter to set the timeout value (in seconds), and -p to set
the ping rate (number of seconds between pings).

Signed-off-by: Timur Tabi <timur@codeaurora.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>