realtek: add 2500base-x patch sequence.

realtek: add 2500base-x patch sequence.

Adds the SerDes patch sequence for 2500base-x to improve the support of
devices with minimal bootloaders (like BootBase). The sequences were
imported from [1] for even lanes and [2] for odd lanes.

[1] https://github.com/ddejean/dms-1250-oss-release/blob/main/sdk/sdk_rtk_switch/rtk-sdk/src/dal/longan/dal_longan_construct.c#L641
[2] https://github.com/ddejean/dms-1250-oss-release/blob/main/sdk/sdk_rtk_switch/rtk-sdk/src/dal/longan/dal_longan_construct.c#L664

Signed-off-by: Damien Dejean <dam.dejean@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/19834
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>