net: dsa: b53: convert to phylink_pcs

Convert B53 to use phylink_pcs for the serdes rather than hooking it
into the MAC-layer callbacks.

Fixes: 81c1681c

 ("net: dsa: b53: mark as non-legacy")
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>