Add Ethernet 1000BASE-X support for PPC4xx

This patch adds a new switch: "CONFIG_PHY_DYNAMIC_ANEG".  When this symbol
is defined, the PHY will advertise it's capabilities for autonegotiation
based on the capabilities shown in the PHY's status registers, including
1000BASE-X.  When "CONFIG_PHY_DYNAMIC_ANEG" is not defined, the PHY will
advertise hard-coded capabilities, as before.

Signed-off-by: Larry Johnson <lrj@acm.org>