realtek: pcs: rtl931x: use generic CMU configuration

realtek: pcs: rtl931x: use generic CMU configuration

The current CMU setup was just copied and slightly adjusted from the
SDK, lacks functionality and logic and doesn't cover all cases we need
(same in the SDK due to multiple reasons). The existing implementation
for RTL930x covers all that and can be reused for RTL931x. Previous
patches made this generic and now we can add the remaining missing
pieces to actually use it for RTL931x. This only includes
implementations for the few variant-specific actions within the
implementation, linking them properly and calling the CMU configuration.

Drop the old CMU code for RTL931x then since it's not needed anymore.

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>