Add support for supplying bus I/O callbacks when initializing an EROM parser.

[landonf]

Summary

This allows us to use the EROM parser API in cases where the standard
bus space I/O APIs are unsuitable. In particular, this will enable parsing
of the device enumeration table directly from bhndb(4) drivers, prior to
full attach and configuration of the bridge.

Depends on D12392

Test Plan

Tested against both bcma(4) and siba(4) desktop (BCM4331,
BCM4321, BCM4312) and MIPS (SENTRY5, BCM) targets.