realtek: pcs: rtl931x: import SerDes setup code from PHY driver

realtek: pcs: rtl931x: import SerDes setup code from PHY driver

Let's start this transition with RTL931X.

Import all functions starting with 'rtl931x_' or 'rtsds_931x' from PHY
driver into the PCS driver, rename all functions to match a common
naming scheme and adjust signature, helper calls and function calls
accordingly to make it work within the PCS driver.

This is just copy&paste and tries to do only mandatory adjustments. The
code will be refactored in succeeding commits.

Also remove 'unused' attribute from helpers as they are used now.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/20369
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>