Add support for GigE PHYs in MII support library.

This patch allows GigE drivers to use the MII library the same way
10/100 drivers do.

Since the MII library is already used by lots of network drivers and the
GigE MII register bit definitions were reserved when many 10/100 PHYs
were designed, the new GigE registers are accessed only if a driver
specifically enables it. Existing 10/100 drivers should see no behavior
differences with this change.
Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>