net: mscc: ocelot: expose generic phylink_mac_config routine

The ocelot-switch driver can utilize the phylink_mac_config routine. Move
this to the ocelot library location and export the symbol to make this
possible.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Signed-off-by: David S. Miller <davem@davemloft.net>