[SPM] tidying common code to tftf and cactus

This patch moves the code used to test SPM functionality, not explicitly
described in FF-A specification, from ffa_helpers to spm_common which is
built for both tftf and cactus.

Signed-off-by: Max Shvetsov <maksims.svecovs@arm.com>
Change-Id: I461efad977cc4d02701feea7b2215a61453716ef
16 files changed