selftests: watchdog: Add optional file argument

Some systems have multiple watchdog devices where the first device
registered is assigned to the /dev/watchdog device file. In order
to test other watchdog devices, add an optional file argument for
selecting non-default watchdog devices for testing.

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