Add support for multiple GICv3 ITS devices. For this we add sc_irq_base
and sc_irq_length to the softc to handle the base number of IRQs available,
make gicv3_get_nirqs return the number of available interrupt IDs, and
limit which CPUs we send interrupts to based on the numa domain.

The last point is only strictly needed on a dual socket ThunderX where we
are unable to send MSI/MSI-X interrupts between sockets.

Sponsored by: DARPA, AFRL