dt-bindings: net: pcs: add bindings for Renesas RZ/N1 MII converter

This MII converter can be found on the RZ/N1 processor family. The MII
converter ports are declared as subnodes which are then referenced by
users of the PCS driver such as the switch.

Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>