test(interrupts): exercise ns interrupt being signaled to SP

This patch adds a tftf based test to exercise non-secure interrupt
handling being signaled to the Cactus SP by the SPMC while processing
a direct message.

A detailed description is provided in the comment leading the test
function.

Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
Change-Id: I2f8511135470d2ca8f567532a1cb7c4ae374af3f
3 files changed