feat(ff-a): create function helper to setup FF-A mailbox

Fix test dependencies issue when SPM test is used individually.
Provide an API function from SPM test suite to initialize the FF-A
mailbox and enable FF-A based message with SP.

Signed-off-by: Nabil Kahlouche <nabil.kahlouche@arm.com>
Change-Id: I246491907f1641c47937a9a1c91cfd4a9b8bfe20
2 files changed