SPM: Add backend operation 'replying'

SFN and IPC have different message replying mechanisms:

- SFN replies to the client by SPM after messaging is performed;
  while IPC partition calls 'psa_reply' initiatively to reply to
  the client.

Abstract 'replying' to decouple the backend-specific operations
out of common FFM logic.

Change-Id: Ie28a0eaa1b37dfe458328dfb6ba298a0bf979c26
Signed-off-by: Ken Liu <Ken.Liu@arm.com>
3 files changed