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>
diff --git a/Documentation/watchdog/watchdog-api.txt b/Documentation/watchdog/watchdog-api.txt
index b3a701f..0e62ba3 100644
--- a/Documentation/watchdog/watchdog-api.txt
+++ b/Documentation/watchdog/watchdog-api.txt
@@ -37,7 +37,7 @@
 the watchdog is pinged within a certain time, this time is called the
 timeout or margin.  The simplest way to ping the watchdog is to write
 some data to the device.  So a very simple watchdog daemon would look
-like this source file:  see Documentation/watchdog/src/watchdog-simple.c
+like this source file:  see samples/watchdog/watchdog-simple.c
 
 A more advanced driver could for example check that a HTTP server is
 still responding before doing the write call to ping the watchdog.