acpica: Extract _OSC parsing to a common file

This will be used by pci_host_generic_acpi.c so needs to be in a
common location.

Reviewed by: imp, jhb
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D48044

(cherry picked from commit ba1904937d9ae0539e39001467a1519b17177118)