selftests/watchdog: change to print reset reason info.

The test prints reset reason when WDIOC_GETBOOTSTATUS returns
a 0 flag value and all other cases simply say watchdog.

Parse the flag values returned by WDIOC_GETBOOTSTATUS and print
corresponding reset reason for each WDIOF_* boot status.

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>