realtek: pcs: rtl931x: configure SerDes-backing MAC groups

realtek: pcs: rtl931x: configure SerDes-backing MAC groups

RTL931x organizes MACs into 12 groups (one per SerDes) that must be
explicitly enabled before link establishment. Without initialization,
link may fail or packets may be corrupted, especially in USXGMII/XSGMII
modes. This is the case for devices which lack initialization by the
bootloader.

Simply enable all MACs in all groups by writing 0xffffffff to the
registers. Unused MACs and reserved bits are harmless, avoiding complex
logic to always set only needed MACs.

This is placed in the PCS driver since the MAC groups are assigned per
SerDes and the DSA driver lacks SerDes awareness (on purpose)

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22088
Signed-off-by: Robert Marko <robimarko@gmail.com>