physmem: add ram0 pseudo-driver

Its purpose is to reserve all I/O space belonging to physical memory
from nexus, preventing it from being handed out by bus_alloc_resource()
to callers such as xenpv_alloc_physmem(), which looks for the first
available free range it can get. This mimics the existing pseudo-driver
on x86.

If needed, the device can be disabled with hint.ram.0.disabled="1" in
/boot/device.hints.

Reviewed by: imp
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D32343