mv643xx_eth: use auto phy polling for configuring (R)(G)MII interface

The mv643xx_eth hardware has a provision for polling the PHY's
MII management registers to obtain the (R)(G)MII interface speed
(10/100/1000) and duplex (half/full) and pause (off/symmetric)
settings to use to talk to the PHY.

The driver currently does not make use of this feature.  Instead,
whenever there is a link status change event, it reads the current
link parameters from the PHY, and programs those parameters into
the mv643xx_eth MAC by hand.

This patch switches the mv643xx_eth driver to letting the MAC
auto-determine the (R)(G)MII link parameters by PHY polling, if there
is a PHY present.  For PHYless ports (when e.g. the (R)(G)MII
interface is connected to a hardware switch), we keep hardcoding the
MII interface parameters.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>