cantrip/irq: support connecting multiple irq's to a single ntfn

Expand the irq connection syntax to support routing multiple irq's to
one endpoint. This is useful for reducing the overhead (thread, stack,
ipc_buffer, etc) for low-duty/low-priority irq's.  Syntax is:

connection cantripIRQ <name>(from <srcA>, from <srcB>, ..., to <dest>);

One thread is created for <dest>. The thread's run method should wait
on the master notification object (<dest>_NOTIFICATION) and then check
the received badge/bitmask to find out which src's were triggered.
Source connections are badged with a bitmask value constructed from
the position in the connection list; e.g. <srcA> = 1<<0, <srcB> = 1<<1,
etc. so a received bitmask of 3 indicates <srcA> & <srcB> were triggered.

Note all IRQ's are badged, even when there is a dedicated thread.

Change-Id: I82262f4a5c399200e1d0acee937644e41a4d3312