net: phylink: add struct phylink_pcs

Add a way for MAC PCS to have private data while keeping independence
from struct phylink_config, which is used for the MAC itself. We need
this independence as we will have stand-alone code for PCS that is
independent of the MAC.  Introduce struct phylink_pcs, which is
designed to be embedded in a driver private data structure.

This structure does not include a mdio_device as there are PCS
implementations such as the Marvell DSA and network drivers where this
is not necessary.

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>