pci_dw: Detect number of outbound regions automatically

Currently we use the num-viewports property to decide how many outbound
regions there are we can use, defaulting to 2. However, Linux has
stopped using that and so it no longer appears in new device trees, such
as for the SiFive FU740. Instead, it's possible to just probe the
hardware directly.

Reviewed by: mmel
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31030