feat(ff-a): add helpers for indirect message

Add the header structures for the rxtx buffers, other
helper macros, and the FFA_MSG_SEND2 ABI caller.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: Ie5c735aa0b33dbf0288b6494362b6e7dc84c6db7
3 files changed