Add the start of INTRNG support for ACPI.
This adds a new acpi_bus interface with a map_intr method. This is similar
to the Open Firmware map_intr method and allows us to create the needed
mapping from ACPI space to INTRNG space.
Obtained from: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8617