MFC: r227687, r228290 (partial)

MFC: r227687, r228290 (partial)

- Add a hint.miibus.X.phymask hint, allowing do individually exclude PHY
  addresses from being probed and attaching something including ukphy(4)
  to it. This is mainly necessarily for PHY switches that create duplicate
  or fake PHYs on the bus that can corrupt the PHY state when accessed or
  simply cause problems when ukphy(4) isolates the additional instances.
- Change miibus(4) to be a hinted bus, allowing to add child devices via
  hints and to set their attach arguments (including for automatically
  probed PHYs). This is mainly needed for PHY switches that violate IEEE
  802.3 and don't even implement the basic register set so we can't probe
  them automatically. However, the ability to alter the attach arguments
  for automatically probed PHYs is also useful as for example it allows
  to test (or tell a user to test) new variant of a PHY with a specific
  driver by letting an existing driver attach to it via manipulating the
  IDs without the need to touch the source code or to limit a Gigabit
  Ethernet PHY to only announce up to Fast Ethernet in order to save
  energy  by limiting the capability mask. Generally, a driver has to
  be hinted via hint.phydrv.X.at="miibusY" and hint.phydrv.X.phyno="Z"
  (which already is sufficient to add phydrvX at miibusY at PHY address
  Z). Then optionally the following attach arguments additionally can
  be configured:
  hint.phydrv.X.id1
  hint.phydrv.X.id2
  hint.phydrv.X.capmask
- Some minor cleanup.

Reviewed by:	adrian, ray

git-svn-id: svn://svn.freebsd.org/base/stable/8@230710 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f