Add support for hw.physmem tunable for ARM/ARM64/RISC-V platforms

hw.physmem tunable allows to limit number of physical memory available to the
system. It's handled in machdep files for x86 and PowerPC. This patch adds
required logic to the consolidated physmem management interface that is used by
ARM, ARM64, and RISC-V.

Submitted by: Klara, Inc.
Reviewed by: mhorne
Sponsored by: Ampere Computing
Differential Revision: https://reviews.freebsd.org/D27152