AnyXmlNode builders

Add builders to yang-data-impl to be able to implement
DataNormalizationOperation.AnyXmlNormalization.normalize() from sal-common-impl

Change-Id: I9033b9a5a228a552a4abc6230d6e5ce63d49fc78
Signed-off-by: Jozef Gloncak <jgloncak@cisco.com>
Merge "AnyXmlNode builders"