ath79: add QCA955x SGMII link loss workaround

ath79: add QCA955x SGMII link loss workaround

This commit adds a workaround for the loss of the SGMII link observed on
the QCA955x generation of SoCs. The workaround originates part from the
U-Boot source code, part from the implementation from AVM found in the
GPL tarball for the AVM FRITZ!WLAN Repeater 450E.

The bug results in a stuck SGMII link between the PHY device and the SoC
side. This has only been observed with the Atheros AR8033 PHY and most
likely all devices using such combination are affected.

It is worked around by reading a hidden SGMII status register and
issuing a SGMII PHY reset until the link becomes useable again.

Signed-off-by: David Bauer <mail@david-bauer.net>