docs(ff-a): update documentation of FF-A interfaces

- Overview of FF-A v1.1 notifications feature, and list of all
the new related interface.
- FFA_RXTX_UNMAP now implemented, so provided description.
- FF-A v1.1 interfaces documented: FFA_SPM_ID_GET and
FFA_SECONDARY_EP_REGISTER.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: If40b4d2b2473f81ecfb2ddbf14852c3f10682867
1 file changed