net: phylink: add interface to configure clause 22 PCS PHY

Add an interface to configure the advertisement for a clause 22 PCS
PHY, and set the AN enable flag in the BMCR appropriately.

Reviewed-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>