nvdimm(4): Extract ACPI root bus driver

No functional change intended.

The intent is to add a "legacy" e820 pmem newbus bus for nvdimm device in a
subsequent revision, and it's a little more clear if the parent buses get
independent source files.

Quite a lot of ACPI-specific logic is left in nvdimm.c; disentangling that
is a much larger change (and probably not especially useful).

Reviewed by: kib
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D21813