doc: sfp-phylink: update the porting guide with PCS handling

Now that phylink has a comprehensive PCS support, update the porting
guide to explain the process of supporting the PCS configuration. This
also removed outdated references to phylink_config fields that no longer
exists.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>