ARM: dts: r9a06g032-rzn1d400-db: add switch description

Add description for the switch, GMAC2 and MII converter. With these
definitions, the switch port 0 and 1 (MII port 5 and 4) are working on
RZ/N1D-DB board.

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