realtek: add 10G_QXGMII serdes mode support for RTL930x

realtek: add 10G_QXGMII serdes mode support for RTL930x

In Realtek implementation USXGMII is divided in submodes:
 - USXGMII_SX: 10G single link, equivalent of PHY_INTERFACE_MODE_USXGMII
 - USXGMII_DX: 10G two links (2*5G ?),
 - USXGMII_QX: 10G four links, presumably 4*2.5G, used with the RTL8224,
   equivalent of PHY_INTERFACE_MODE_10G_QXGMII.

This CL adds the 10_GQXGMII modes to the RTL930x implementation. In
particular the "mode set" function is extended to support both simple
mode set, and force mode set depending on the mode according to
dal_longan_sds_mode_set [1].

[1] https://github.com/ddejean/dms-1250-oss-release/blob/main/sdk/sdk_rtk_switch/rtk-sdk/src/dal/longan/dal_longan_sds.c#L1746

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>