arm/gic: Detect broken configurations

Some virtualization platforms provide broken configurations. There
is a GIC interrupt controller, however accessing the CPU interface
registers leads to an external data abort. As these are needed to
handle interrupts we are unable to boot further.

Detect this misconfiguration and panic to tell the user the issue.

Reviewed by: emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D54832