ar71xx: fix QCA955X SGMII link loss

ar71xx: fix QCA955X SGMII link loss

The QCA955X is affected by a hardware bug which causes link-loss of the
SGMII link between SoC and PHY. This happens on change of link-state or
speed.

It is not really known what causes this bug. It definitely occurs when
using a AR8033 Gigabit Ethernet PHY.

Qualcomm solves this Bug in a similar fashion. We need to apply the fix
on a per-device base via platform-data as performing the fixup work will
break connectivity in case the SGMII interface is connected to a Switch.

This bug was first proposed to be fixed by Sven Eckelmann in 2016.
 https://patchwork.ozlabs.org/patch/604782/

Based-on-patch-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>
Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit f4f99ec9737c653815268f2efad0210caaa32e2d)