Improve mac+phy configuration so that hints can be used to describe

Improve mac+phy configuration so that hints can be used to describe
layouts different than the defaults:
o hint.npe.0.mac="A", "B", etc. specifies the window for MAC register accesses
o hint.npe.0.mii="A", "B", etc. specifies PHY registers
o hint.npe.1.phy=%d specifies the PHY to map to a port

This allows devices like NSLU to be setup w/o code changes and will
also be used for forthcoming support for more Avila boards.

Reviewed by:	imp
MFC after	1 week