feat(spmc): add helper function to obtain endpoint mailbox

Add a helper function to obtain the relevant mailbox buffers
depending on which entity was last run. This will be used in
subsequent functionality to populate requested information in
the callers RX buffer.

Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: I85959ced4d1454be05a7b3fb6853ed3ab7f0cf3e
2 files changed