net: mscc: ocelot: expose serdes configuration function

During chip initialization, ports that use SGMII / QSGMII to interface to
external phys need to be configured on the VSC7513 and VSC7514. Expose this
configuration routine, so it can be used by DSA drivers.

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