selftests: watchdog: accept multiple params on command line

Watchdog drivers are not required to retain programming information,
such as timeouts, after the watchdog device is closed.  Therefore,
the watchdog test should be able to perform multiple actions after
opening the watchdog device.

For example, to set the timeout to 10s and ping every 5s:

	watchdog-test -t 10 -p 5 -e

Also, display the periodic decimal point only if the keep-alive call
succeeds.

Signed-off-by: Timur Tabi <timur@codeaurora.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>