net: phylink: add pcs_pre_config()/pcs_post_config() methods

Add hooks that are called before and after the mac_config() call,
which will be needed to deal with errata workarounds for the
Marvell 88e639x DSA switches.

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