fix(doc): document missing RMM-EL3 runtime services

This patch adds documentation for the missing RMM-EL3
runtime services:

* RMM_RMI_REQ_COMPLETE
* RMM_GTSI_DELEGATE
* RMM_GTSI_UNDELEGATE

This patch also fixes a couple of minor bugs on return codes
for delegate/undelegate internal APIs.

Signed-off-by: Javier Almansa Sobrino <javier.almansasobrino@arm.com>
Change-Id: Ic721005e7851e838eebaee7865ba78fadc3309e4