docs(rmmd): document EL3-RMM Interfaces

This patch documents the RMM-EL3 Boot and runtime interfaces.

Note that for the runtime interfaces, some services are not
documented in this patch and will be added on a later doc patch.

These services are:

* RMMD_GTSI_DELEGATE
* RMMD_GTSI_UNDELEGATE
* RMMD_RMI_REQ_COMPLETE

Signed-off-by: Javier Almansa Sobrino <javier.almansasobrino@arm.com>
Change-Id: I8fcc89d91fe5a334c2f68c6bfd1fd672a8738b5c