net: dsa: rzn1-a5psw: add Renesas RZ/N1 advanced 5 port switch driver

Add Renesas RZ/N1 advanced 5 port switch driver. This switch handles 5
ports including 1 CPU management port. A MDIO bus is also exposed by
this switch and allows to communicate with PHYs connected to the ports.
Each switch port (except for the CPU management ports) is connected to
the MII converter.

This driver includes basic bridging support, more support will be added
later (vlan, etc).

Suggested-by: Jean-Pierre Geslin <jean-pierre.geslin@non.se.com>
Suggested-by: Phil Edworthy <phil.edworthy@renesas.com>
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>