Convert arm's physmem interface to MI code

[mhorne]

Summary

The arm_physmem interface found in arm's MD code provides a convenient
set of routines for adding/excluding physical memory regions and
initializing important kernel globals such as Maxmem, realmem,
phys_avail[], and dump_avail[]. It is especially convenient for FDT
systems, since we can use FDT parsing functions and pass the result
directly to one of these physmem routines. This interface is already in
use on arm and arm64, and can be used to simplify this early
initialization on RISC-V as well.

This requires only a couple trivial changes:

• Move arm_physmem_kernaddr to arm/machdep.c. It is unused on arm64, and manipulated entirely in arm MD code.
• Convert arm32_btop/arm64_btop to atop. This is equivalently defined on all architectures.
• Drop the "arm" prefix.

Test Plan

Compile and boot on armv7
Compile and boot on riscv
Compile on arm64

[manu]

LGTM

[nick]

I can confirm this boots on our RISC-V platform.

[emaste]

looks reasonable to me