phy: lan966x: add support for QUSGMII

Makes so that the serdes driver also takes QUSGMII in consideration.
It's configured exactly as QSGMII as far as the serdes driver is
concerned.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>