xen: notify ACPI about SCI override

If the SCI is remapped to a non-ISA global interrupt notify the ACPI
subsystem about the override.

Reported by: David P. Discher <dpd@dpdtech.com>
Sponsored by: Citrix Systems R&D


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275641 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f