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
diff --git a/docs/components/realm-management-extension.rst b/docs/components/realm-management-extension.rst
index 5fa5140..ea921fc 100644
--- a/docs/components/realm-management-extension.rst
+++ b/docs/components/realm-management-extension.rst
@@ -73,6 +73,14 @@
 world. It initializes the RMM and handles Realm Management Interface (RMI)
 SMC calls from Non-secure and Realm worlds.
 
+There is a contract between RMM and RMMD that defines the arguments that the
+former needs to take in order to initialize and also the possible return values.
+This contract is defined in the RMM Boot Interface, which can be found at
+:ref:`rmm_el3_boot_interface`.
+
+There is also a specification of the runtime services provided by TF-A
+to RMM. This can be found at :ref:`runtime_services_and_interface`.
+
 Test Realm Payload (TRP)
 *************************
 TRP is a small test payload that runs at R-EL2 and implements a subset of