FF-A: Accessors for arguments from FF-A calls

Defined some accessors for arguments from FF-A calls, namely for
func id, error code, and direct message destination/source.
This should help make consistent how they were being handled,
enforcing also adequate type checking.
Replace the accesses to those FF-A arguments with respective wrapper
calls.

Change-Id: I99d8e77f3b24728c30eafa3e76a830246790ec5f
Signed-off-by: J-Alves <joao.alves@arm.com>
9 files changed