realtek: pcs: rtl931x: add sequence for LC PLL

realtek: pcs: rtl931x: add sequence for LC PLL

The RTL931X CMU code was only capable of setting up ring PLL. This is
fine so far as most modes use this PLL type. Other modes are not handled
by the code, neither here nor in the SDK. Though, the SDK has the needed
sequence to setup the LC PLL. Using LC PLL seems to be handled somewhere
else.

Include the small sequence from the SDK to have it, though not used yet.
This could be helpful for further development which goes beyond the SDK.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21707
Signed-off-by: Robert Marko <robimarko@gmail.com>