ppc: qemu: Enable eTSEC support

ppc: qemu: Enable eTSEC support

QEMU ppce500 target can dynamically instantiate an eTSEC device
if "-device eTSEC" is given to QEMU. This commit enables eTSEC
driver and the required fixed PHY driver to create a usable
network configuration using eTSEC.

Unlike a real world 85xx board that usually stores the eTSEC MAC
address in an EEPROM, CONFIG_NET_RANDOM_ETHADDR is required for
QEMU otherwise U-Boot ethernet initialization complains no valid
ethernet address is set.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>