[kernel] Add KernelPhysicalAddressOf helper

Consolidate a slightly arcane, repeated pattern into a subroutine.

Change-Id: I5cc0b20ac609c204a0066a9324b6406e7d212c2b
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1288385
Reviewed-by: Nick Maniscalco <maniscalco@google.com>
Commit-Queue: Roland McGrath <mcgrathr@google.com>
Fuchsia-Auto-Submit: Roland McGrath <mcgrathr@google.com>
Reviewed-by: Joshua Seaton <joshuaseaton@google.com>
Reviewed-by: Travis Geiselbrecht <travisg@google.com>