net: phylink: add pcs_enable()/pcs_disable() methods

Add phylink PCS enable/disable callbacks that will allow us to place
IEEE 802.3 register compliant PCS in power-down mode while not being
used.

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