ARM: dts: lan966x: add serdes node

Add the SerDes node. On the LAN966x SoC these SerDes are used to connect
network PHYs.

By default, that node is disabled.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Tested-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220502224127.2604333-12-michael@walle.cc


Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>