Update DocBook validating schema to 5.1CR1

Update the RelaxNG schema used by tools/validate.py to 5.1CR1.

This schema is used to validate individual files. The new schema
should allow any entity to be at the root. This should eliminate
false negative xinclude-related validation errors.

Change-Id: I24cfe9bf75fd776eff11fcd1753ded0e7e8a39bd
Merge "Update DocBook validating schema to 5.1CR1"