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

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 allow us to parse
the device enumeration table directly from bhndb(4) drivers, prior to
full attach and configuration of the bridge.

Approved by: adrian (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D12510