hw/net: add MII definitions

This adds comments on the Basic mode control and status registers bit
definitions. It also adds a couple of bits for 1000BASE-T and the
RealTek 8211E PHY for the FTGMAC100 model to use.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>