SPCI is now called PSA FF-A

SPCI is renamed as PSA FF-A which stands for Platform Security
Architecture Firmware Framework for A class processors.
This patch replaces the occurrence of SPCI with PSA FF-A(in documents)
or simply FFA(in code).

Change-Id: I4ab10adb9ffeef1ff784641dfafd99f515133760
Signed-off-by: J-Alves <joao.alves@arm.com>