trusty: Add support for doorbell interrupt

Support using a single (per-cpu) doorbell interrupt to signal that there
is an interrupt pending for trusty. This can be used instead of
registering a linux interrupt handler for every trusty interrupt.

Bug: 122357256
Change-Id: I146f04ed45ea466cce3c571c93f8d694969b90ba
Signed-off-by: Arve Hjønnevåg <arve@android.com>