realtek: pcs: rtl930x: split pll config

realtek: pcs: rtl930x: split pll config

Split up PLL configuration of RTL930x in the two distinct actions of
configuring the PLL itself (aka setting its speed, etc.) and selecting
which PLL is used by a SerDes.

It was found that for both RTL930x and RTL931x, PLL configuration can be
combined while selecting the PLL a SerDes uses differs and needs to be
implemented variant-specific.

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>