net: dsa: felix: allow serdes configuration for dsa ports

Ports for Ocelot devices (VSC7511, VSC7512, VSC7513 and VSC7514) support
external phys. When external phys are used, additional configuration on
each port is required to enable QSGMII mode and set external phy modes.

Add a configurable hook into these routines, so the external ports can be
used.

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