Limit devicekey example to only K64F

Lines up devicekey example with nvstore example

Author: Cruz Monrreal

tools/test/examples/examples.json

@@ -258,7 +258,7 @@
         "mbed": [],
         "test-repo-source": "github",
         "features" : [],
-        "targets" : [],
+        "targets" : ["K64F"],
         "toolchains" : [],
         "exporters": [],
         "compile" : true,