Limit the number of CPUs in the gicv1/2 driver

The GICv2 can only send IPIs to 8 CPUs. Because of this it should only
be in machines with no more than 8 cores.

Create a new macro to hold this limit to reduce the size of the softc.

Reviewed by: emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D41322