dt-bindings: net: ethernet-controller: add PSGMII mode

Add a new PSGMII mode which is similar to QSGMII with the difference being
that it combines 5 SGMII lines into a single link compared to 4 on QSGMII.

It is commonly used by Qualcomm on their QCA807x PHY series.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>