net/cadence_gem: Implement mac level loopback mode

Cadence GEM has a MAC level loopback mode. Implement. Use the same basic
operation as the already implemented PHY loopback.

Reported-by: Deepika Dhamija <deepika@xilinx.com>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 3a0baf1b6b2fc1be638bdf1a37408ec38988e970.1386136219.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
