net: phylink: pcs: add 802.3 clause 45 helpers

Implement helpers for PCS accessed via the MII bus using 802.3 clause
45 cycles for 10GBASE-R. Only link up/down is supported, 10G full
duplex is assumed.

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