Add xgmac ethernet model

This adds very basic support for the xgmac ethernet core. Missing things
include:

- statistics counters
- WoL support
- rx checksum offload
- chained descriptors (only linear descriptor ring)
- broadcast and multicast handling

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Mark Langsdorf <mark.langsdorf@calxeda.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
