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