pci_dw: Support modern "unroll" iATU mode

This supersedes the old legacy mode where a viewport register was used
to mux multiple regions behind a single set of registers, and is used on
the SiFive FU740.

Reviewed by: mmel
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31029

(cherry picked from commit f240dfff229d1f1ff502f59901ef2b9364ca55d9)