On SPARCbook systems, the ledma device node has a `cable-selection' property

On SPARCbook systems, the ledma device node has a `cable-selection' property
specifying which media the on-board interface uses. We already query it to
set up proper register values; extend this to be able to pass a default
media to the le(4) child.

This makes SPARCbook system default to AUI without needing for a manual media
change.

tested by sebastia@