topology: pre-process-class: add funcion to get the name of the unique attribute...

topology: pre-process-class: add funcion to get the name of the unique attribute in a class

Every class must have a unique attribute that will be used
to instantiate the object. The value provided for this
attribute must be unique within the same alsaconf node for
objects of the same class. Add a helper function to get the
name of the attribute that must have a unique value in the
object instance.

For example, when instantiating 2 buffer widgets within a pipeline,
they must be given unique instance attribute values as:
Object.Widget.buffer.0{} and Object.Widget.buffer.1{}.

Signed-off-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>