refactor(rme): reorg existing RMMD EL3 service FIDs

This patch reworks the GTSI service implementation in RMMD
such that it is made internal to RMMD. This rework also
lays the ground work for additional RMMD services which
can be invoked from RMM.

The rework renames some of the FID macros to make it
more suited for adding more RMMD services. All the RMM-EL31
service SMCs are now routed via rmmd_rmm_el3_handler().

Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Change-Id: Ic52ca0f33b79a1fd1deefa8136f9586b088b2e07