pcib: Refine handling of resources allocated from bridge windows

Fix a long-standing layering violation in the original NEW_PCIB code
by not passing suballocated resources up to the parent bus for
activation and mapping. Instead, handle activation and mapping of
sub-allocated resources in this driver. When mapping resources,
request a mapping from a suitable sub-region of the resource allocated
from the parent bus for the associated bridge window.

Note that this does require passing RF_ACTIVE (with RF_UNMAPPED) when
allocating bridge window resources from the parent.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D43690