nvdimm(4): Add nvdimm_e820 pseudo-bus

nvdimm_e820 is a newbus pseudo driver that looks for "legacy" e820 PRAM
spans and creates ordinary-looking SPA devfs nodes for them
(/dev/nvdimm_spaN).

As these legacy regions lack real NFIT SPA regions and namespace
definitions, they must be administratively sliced up externally using
device.hints. This is similar in purpose to the Linux memmap= mechanism.

It is assumed that systems with working NFIT tables will not have any use
for this driver, and that that will be the prevailing style going forward,
so if there are no explicit hints provided, this driver does not
automatically create any devices.

Reviewed by: kib (previous version)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D21885