net: pcs: xpcs: add support for sgmii with no inband AN

In fixed-link use cases, the XPCS can disable the clause 37 in-band
autoneg process, disable the "Automatic Speed Mode Change after CL37 AN"
setting, and force operation in a speed dictated by management.

Add support for this operating mode.

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