samples: move watchdog example code from Documentation

Move watchdog examples to samples and remove it from Documentation
Makefile. Create a new Makefile to build watchdog. It can be built
from top level directory or from watchdog directory:

Run make -C samples/watchdog or cd samples/watchdog; make

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>