Support fixed size, variable location acpi resources

These have been found in some Arm ACPI tables generated by edk2, e.g.
when describing the pl011 uart on the Arm AEMv8 model.

Reviewed by: imp, jkim
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31110

(cherry picked from commit 1472117a1e1c40f10b9c3a48788fbf8355588ee8)