irq_flow_handler_t now returns bool

Alter the signature of irq_flow_handler_t to return true for those interrupts
whose handlers were invoked, and false otherwise.  Also rework the actual
handlers, handle_.*_irq, to support the new signature.

Change-Id: I8a50410c477692bbcd39a0fefdac14253602d1f5
Signed-off-by: Iliyan Malchev <malchev@google.com>