NET: Add Ethernet 1000BASE-X support for PPC4xx

This patch adds support for 1000BASE-X to functions "miiphy_speed ()" and
"miiphy_duplex()".  It also adds function "miiphy_is_1000base_x ()", which
returns non-zero iff the PHY registers are configured for 1000BASE-X.  The
"mii info" command is modified to distinguish between 1000BASE-T and -X.

Signed-off-by: Larry Johnson <lrj@acm.org>
Signed-off-by: Ben Warren <bwarren@qstreams.com>