apei: Mark ReadAckRegister resource as shareable

Work around vendors who use the same address for multiple
ReadAckRegisters in their ACPI HEST table. This
allows apei to attach cleanly on Ampere Altra servers.
Note the issue is not specific to Ampere, I've run into
it with at least one other vendor (whose server is not
yet released).

Sponsored by: Netflix
Reviewed by: jhb