realtek: pcs: make rtl930x CMU config generic

realtek: pcs: make rtl930x CMU config generic

Generalize the RTL930x CMU configuration to support RTL931x as well.
Both implementations differ only in minor details, allowing them to
share common code and avoid duplication.

Affected functions are moved up in the code to the 93xx common area and
slightly renamed. Existing variant-specific functions are adjusted too
and assigned to the previously added SerDes operation hooks.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22198
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>