Cactus: decouple exception handling from tftf framework

So far, tftf framework's exception handling was used for Cactus
exceptions also. With new interrupt related tests coming up in
Cactus, we need a separate exception handler code for Cactus.

This patch enables irq/fiq for Cactus and adds placeholder handlers
for them.

Change-Id: Ifd89c4ba8b4491345948bf342540b37fdbc91b8d
Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
5 files changed