docs: ff-a manifest binding to document SPMC

The FF-A manifest binding section was extended
to document the SPMC manifest as well.

The configuration is for the SPMD, which is common
to SPMC implementations.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: If8747a701212cedebc4a9dd66f2040443ce5e30f
1 file changed