feat(cactus): validate NPI injection

Cactus SP can now handle the Notification Pending Interrupt:
- Added `npi_handled` array to track the CPUs that have handled the
NPI. The NPI handler will set the index of related to the respective
core.
- Extended and refactored cactus command CACTUS_NOTIFICATION_GET_CMD,
such that the sender can specify that the NPI is to be validated.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I2d5728ec4fe2d14fb9ba6f0ba5230ef652467f85
4 files changed