MAINTAINERS: add Renesas RZ/N1 switch related driver entry

After contributing the drivers, volunteer for maintenance and add
myself as the maintainer for Renesas RZ/N1 switch related drivers.

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>