Add Anyxml normalized node to yang-data-api.

Proposal of anyxml normalized node interface.
This interface wraps Simple/CompositeNode as inner value.

Change-Id: Ic9c17fa28bc5eb23288162a735d890b4f77938f1
Signed-off-by: Maros Marsalek <mmarsale@cisco.com>
Signed-off-by: Jozef Gloncak <jgloncak@cisco.com>
Merge "Add Anyxml normalized node to yang-data-api."