siba(4): Adopt bcma-compatible mapping of bhnd(4) port/region identifiers.

Maps Sonics/OCP per-core address spaces to bcma(4)-compatible port/region
identifiers.

This permits the use of common address map identifiers in bhnd device
drivers, independent of the underlying interconnect type.

Approved by: re (gjb), adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D6850