net: phy: Move linkmode helpers to somewhere public

phylink has some useful helpers to working with linkmode bitmaps.
Move them to there own header so other code can use them.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>