net: phylink: make QSGMII a valid PHY mode for in-band AN

QSGMII is a SerDes protocol clocked at 5 Gbaud (4 times higher than
SGMII which is clocked at 1.25 Gbaud), with the same 8b/10b encoding and
some extra symbols for synchronization. Logically it offers 4 SGMII
interfaces multiplexed onto the same physical lanes. Each MAC PCS has
its own in-band AN process with the system side of the QSGMII PHY, which
is identical to the regular SGMII AN process.

So allow QSGMII as a valid in-band AN mode, since it is no different
from software perspective from regular SGMII.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>