ARM: dts: r9a06g032: describe MII converter

Add the MII converter node which describes the MII converter that is
present on the RZ/N1 SoC.

Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>