[POWERPC] mpc5200: Allow for fixed speed MII configurations

Various improvements for configuring the MPC5200 MII link from the
device tree:
* Look for 'current-speed' property for fixed speed MII links
* Look for 'fsl,7-wire-mode' property for boards using the 7 wire mode
* move definition of private data structure out of the header file

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Wolfgang Grandegger <wg@grandegger.com>