arm64 gicv3: Use void pointers instead of vm_offset_t

This removes the need for local diffs in CheriBSD where vm_offset_t
is an address and not the same as a uintptr_t. No functional change.

Reviewed by: imp, andrew
Sponsored by: University of Cambridge, Google, Inc.
Differential Revision: https://reviews.freebsd.org/D44341