phy: armada-38x: fix NETA lockup when repeatedly switching speeds

The mvneta hardware appears to lock up in various random ways when
repeatedly switching speeds between 1G and 2.5G, which involves
reprogramming the COMPHY.  It is not entirely clear why this happens,
but best guess is that reprogramming the COMPHY glitches mvneta clocks
causing the hardware to fail.  It seems that rebooting resolves the
failure, but not down/up cycling the interface alone.

Various other approaches have been tried, such as trying to cleanly
power down the COMPHY and then take it back through the power up
initialisation, but this does not seem to help.

It was finally noticed that u-boot's last step when configuring a
COMPHY for "SGMII" mode was to poke at a register described as
"GBE_CONFIGURATION_REG", which is undocumented in any external
documentation.  All that we have is the fact that u-boot sets a bit
corresponding to the "SGMII" lane at the end of COMPHY initialisation.

Experimentation shows that if we clear this bit prior to changing the
speed, and then set it afterwards, mvneta does not suffer this problem
on the SolidRun Clearfog when switching speeds between 1G and 2.5G.

This problem was found while script-testing phylink.

This fix also requires the corresponding change to DT to be effective.
See "ARM: dts: armada-38x: fix NETA lockup when repeatedly switching
speeds".

Fixes: 14dc100b4411 ("phy: armada38x: add common phy support")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/E1jxtRj-0003Tz-CG@rmk-PC.armlinux.org.uk
Signed-off-by: Vinod Koul <vkoul@kernel.org>

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