Add a gic interface to allocate MSI interrupts

The previous update to handle the gicv2m as a child of the gicv3 driver
assumed there was only a single gicv2m child. On some hardware there
are multiple children. Support this by removing the mbi ivars and
adding a new interface to handle MSI allocation in a given range.

Tested by: mw, trasz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32224