Implement stubs for ACPI PCI routines

ACPI driver requires special functions to be provided by machdep code.
Add temporary stubs to satisfy the compiler when both "pci" and "acpi"
are enabled in the kernel configuration file.

Reviewed by: andrew
Obtained from: Semihalf
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3028