net: introduce a helper to determine whether to use in-band autoneg

Certain serial SERDES protocols like 1000base-x, 2500base-x, SGMII,
USXGMII can operate either in a mode where the PHY (be it on-board or
inside an SFP module) passes the link parameters (speed, duplex, pause)
to the MAC through in-band through control words standardized by IEEE
802.3 clause 37, or in a mode where the MAC must configure (force) its
link parameters based on information obtained out-of-band (MDIO reads,
guesswork etc).

In Linux, the OF node property named "managed" is parsed by the phylink
framework, and the convention is that if a driver uses phylink, then the
presence of this property means that in-band autoneg should be enabled,
otherwise it shouldn't.

To be compatible with the OF node bindings of drivers that use phylink
in Linux, introduce parsing support for this property in U-Boot too.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>