net: phylink: pcs: add 802.3 clause 22 helpers

Implement helpers for PCS accessed via the MII bus using 802.3 clause
22 cycles, conforming to 802.3 clause 37 and Cisco SGMII specifications
for the advertisement word.

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