phylink: add helper for configuring 2500BaseX modes

Add a helper for MAC drivers to use in their validate callback to deal
with 2500BaseX vs 1000BaseX modes, where the hardware supports both
but it is not possible to automatically select between them.

This helper defaults to 1000BaseX, as that is the 802.3 standard, and
will allow users to select 2500BaseX either by forcing the speed if
AN is disabled, or by changing the advertising mask if AN is enabled.
Disabling AN is not recommended as it is only the speed that we're
interested in controlling, not the duplex or pause mode parameters.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>