realtek: add serdes mapping for rtl930x

realtek: add serdes mapping for rtl930x

On the RTL930x series the serdes #3 is backed by serdes #10 when pages
0, 1, 2 or 3 are accessed [1]. This changeset modifies the sds mapping
function from a single implementation for the 3 families to one
implementation per chip family. In particular it implements the mapping
required for the rtl930x one.

[1] https://github.com/ddejean/dms-1250-oss-release/blob/main/sdk/sdk_rtk_switch/rtk-sdk/src/dal/longan/dal_longan_sds.c#L624

Signed-off-by: Damien Dejean <dam.dejean@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/20472
Signed-off-by: Robert Marko <robimarko@gmail.com>