Merge changes from topic "el3_direct_msg" into integration

* changes:
  docs(spm): document new build option
  feat(fvp): spmd logical partition smc handler
  feat(fvp): add spmd logical partition
  feat(spmd): get logical partitions info
  feat(spmd): add partition info get regs
  refactor(ff-a): move structure definitions
  feat(spmd): el3 direct message API
  feat(spmd): add spmd logical partitions