mii: Add helpers for parsing SGMII auto-negotiation

Typically a MAC PCS auto-configures itself after it receives the
negotiated copper-side link settings from the PHY, but some MAC devices
are more special and need manual interpretation of the SGMII AN result.

In other cases, the PCS exposes the entire tx_config_reg base page as it
is transmitted on the wire during auto-negotiation, so it makes sense to
be able to decode the equivalent lp_advertised bit mask from the raw u16
(of course, "lp" considering the PCS to be the local PHY).

Therefore, add the bit definitions for the SGMII registers 4 and 5
(local device ability, link partner ability), as well as a link_mode
conversion helper that can be used to feed the AN results into
phy_resolve_aneg_linkmode.

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