pci_host_generic: Support ACPI_IVAR_HANDLE

In the ACPI attachment support the ACPI_IVAR_HANDLE ivar. While here
use the common ivar function to support the common ivars.

Reviewed by: imp, jhb
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D48047

(cherry picked from commit deb36d0c65436d16b04f99cc2a27bd0f3980a6f0)