acpica: add domain ivar

Specialize acpi bus_get_domain method to read ivar.
Execute and cache the _PXM result in the ivar at namespace enumeration
time.

If there is no _PXM, driver for the child can set the ivar to the value
obtained by other means.

Move acpi_get_domain() to acpi_pci.c, it now serves pci buses and
devices on them.

Suggested and reviewed by: jhb
Sponsored by: Advanced Micro Devices (AMD)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D47291