net: dsa: b53: mdio: add support for BCM53134

Add support for the BCM53134 Ethernet switch in the existing b53 dsa driver.
BCM53134 is very similar to the BCM58XX series.

Signed-off-by: Paul Geurts <paul.geurts@prodrive-technologies.com>
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>