IVGCVSW-4400 Backend Counter Registry Functionality

IVGCVSW-4400 Backend Counter Registry Functionality

 * Adding BackendProfilingContext to the MockBackend
 * Made IBackendProfilingContext pure Virtual
 * Added UnitTest using MockBackend for testing Backend Counter Registration
 * Moved Registry of backend counters from Initialize() to AddBackendProfilingContext()
 * Added m_MaxGlobalCounterId to ProfilingService
 * Removed automatic registration of MockBack in BackendRegistry()

Signed-off-by: David Monahan <david.monahan@arm.com>
Change-Id: Ie1c6c31e56d1ac7079d6116ecad041961014aedc