fs_enet: Add support for MPC512x to fs_enet driver

Extend the fs_enet driver to support MPC512x FEC.
Enable it with CONFIG_FS_ENET_MPC5121_FEC option.

Signed-off-by: John Rigby <jcrigby@gmail.com>
Signed-off-by: Piotr Ziecik <kosmo@semihalf.com>
Signed-off-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>