ITS: Implement the top layer of the ITS service

Implements the secure and non-secure top layer of the ITS service,
including a new manifest file, and generates the required veneers.
Updates the partition manifest with added SIDs. It also adds the
necessary CMake files to build the ITS service.

Change-Id: I6d66b01de56145d24a0bcbf0597ca158b6da3386
Signed-off-by: Tudor Cretu <tudor.cretu@arm.com>