On an Ampere Altra system, the physical memory is populated
sparsely within the physical address space, with only about 0.4%
of physical addresses backed by RAM in the range [0, last_pa].

This is causing the vm_reserv_array to be over-sized by a few
orders of magnitude, wasting roughly 5 GiB on a system with
256 GiB of RAM.

The sparse allocation of vm_reserv_array is controlled by defining
VM_PHYSSEG_SPARSE, with the dense allocation still remaining for
platforms with VM_PHYSSEG_DENSE.

Reviewed by:	markj, alc, kib
Approved by:	scottl (implicit)
MFC after:	1 week
Sponsored by:	Ampere Computing, Inc.
Differential Revision:	https://reviews.freebsd.org/D26130