of_mdio: Add "broadcom,bcm5241" to the whitelist.

Some Cavium dev boards have firmware which doesn't supply a proper
ethernet-phy-ieee802.3-c22" compatible property.  Restore these boards
to working order by whitelisting this compatible value.

Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>