SPM: Add mailbox agent support in SFN model

Currently, the mailbox agent only supports IPC model.
This patch enables mailbox agent in SFN model. Unlike
IPC model, the reply request is handled as a sync one.
This patch covers the default dual-core routine and the
RSS routine.

Signed-off-by: Sherry Zhang <sherry.zhang2@arm.com>
Change-Id: I1035d81aa2427d2e58acaa187b44bbcef275426c
17 files changed