Add support for gicv2m as a child of gicv3

On some systems, e.g. Parallels set to host a Linux VM under an M1 Mac,
there is a GICv2m as a child of the GICv3. We previously assumed the
GICv2m was always a child of a GICv2. Fix this by adding the needed
support to the GICv3 driver.

PR: 258136
Reported by: trasz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31768