realtek: pcs: add init_serdes_common hook

realtek: pcs: add init_serdes_common hook

Add a new hook called 'init_serdes_common' to be able to perform
initialisations or anything else subject to all SerDes. This hook is
called in the end of 'rtpcs_probe' after everything else is done.

This is meant primarily to support the transition of RTL83XX from PHY
driver to PCS driver. Thus, it may be removed later again or kept if
there is sufficient need for this.

Signed-off-by: Jonas Jelonek <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20876
Signed-off-by: Hauke Mehrtens <[email protected]>