net: dsa: dsa_loop: add phylink capabilities

Add phylink capabilities for dsa_loop, which I believe being a software
construct means that it supports essentially all interface types and
all speeds.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>