Add a new QUSGMII mode, standing for "Quad Universal Serial Gigabit
Media Independent Interface", a derivative of USGMII which, similarly to
QSGMII, allows to multiplex 4 1Gbps links to a Quad-PHY.

The main difference with QSGMII is that QUSGMII can include an extension
instead of the standard 7bytes ethernet preamble, allowing to convey
arbitrary data such as Timestamps.
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>