realtek: support configuring SerDes auto-negotiation on RTL93xx

realtek: support configuring SerDes auto-negotiation on RTL93xx

There are SFP modules which only work if auto-negotiation is disabled,
like some "OEM SFP-2.5G-T" modules. This also seems to be necessary for
RTL8226/RTL8221B PHYs when using 2500Base-X.

However, currently, it is always enabled, so add support for configuring
it to make these SFP modules and PHYs work.

This also adds locking which should be useful for future extension of
the PCS driver.

Signed-off-by: Jan Hoffmann <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19518
Signed-off-by: Robert Marko <[email protected]>