Convert counter def format to YAML (#976)

* Convert counter def format to YAML

Converts counter definition format to YAML with the
following structure:

```yaml
COUNTER_NAME:
 architectures:
  gfxXX: // Can be more than one, / deliminated if they share idential data
    block: <Optional>
    event: <Optional>
    expression: <optional>
    description: <Optional> // In case per arch notes are needed
  gfxYY:
    ...
 description: General counter desctiption
```

All counters (derived and hardware) are now defined
in the same file for ease of future additions/subtractions.

Removes existing XML parser. Keeps the existing XML
definitions for now (since other tools still rely on
its presence).

.gitmodules

@@ -25,3 +25,6 @@
 [submodule "external/elfio"]
 	path = external/elfio
 	url = https://github.com/serge1/ELFIO.git
+[submodule "external/yaml-cpp"]
+	path = external/yaml-cpp
+	url = https://github.com/jbeder/yaml-cpp.git