[BUG,BISECTED] mvneta: second interface no more usable on mirabox

Hi,

On Mirabox, the second ethernet interface is no more usable on 4.1-rc*
series (no packets coming out of the interface, when using dhclient for
instance). It works as expected on 4.0.

Bisecting the issue, I ended up on 898b2970e2c9 ("mvneta: implement
SGMII-based in-band link state signaling"). Reverting that commit gives 
me back the second interface.

Then, I also tested on a NETGEAR ReadyNAS 104, which is also powered by
the same SoC (Armada 370) and also has two (mvneta-supported) ethernet
interfaces. With an unmodified 4.1-rc8, only one of the two interfaces
is available. Reverting 898b2970e2c9 makes both usable again.

FWIW, mirabox and RN104 ethernet interfaces use RGMII.

Cheers,

a+