gicv3: Use an offset to find the redist registers

To find the redistributor registers use the resource we have already
found and add an offset. This removed the need to create a
per-redistributor resource as it can now be a pointer to the resource
found in attach.

While here check the offset is within the bounds of the resource. Some
ACPI tables list each redistributor as a separate memory range, even
if they are physically contiguous. In this case we may not have each
resource virtually contiguous with neighbouring resources. This can
lead to a data abort when reading past the resource range.

Reviewed by: kevans
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D40263