selftests: watchdog: Add command line option to show watchdog_info

With the new ioctl(WDIOC_GETSUPPORT) call in place, add a command
line option to show the watchdog_info.

Suggested-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: George G. Davis <george_davis@mentor.com>
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>