Add a callback for a serdes setup function to rtpcs_cfg to allow each
SoC variant to define its own SerDes setup routine.
Call the setup_serdes operation in pcs_config if it is defined.
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/20352
Signed-off-by: Robert Marko <robimarko@gmail.com>