Tidying FFA helpers

Moved SP specific functions/macros to "sp_helpers.c/h".
Placed ffa interfaces/symbols shared between tftf and SPs in
"ffa_helpers.c/h".

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I4cf967f87837ce3e7d0f7244f9cc0677f608f9cd
10 files changed