siba(4): Ignore disabled per-core address match entries.

Previously, the address regions described by disabled admatch entries would
be treated as being mapped to the given core; while incorrect, this was
essentially harmless given that the entries describe unused address space
on the few affected devices.

We now perform parsing of per-core admatch registers and interrupt flags in
siba_erom, correctly skip any disabled admatch entries, and use the
siba_erom API in siba_add_children() to perform enumeration of attached
cores.