net: dsa: microchip: support for "ethernet-ports" node

The dsa.yaml device tree binding allows "ethernet-ports" (preferred) and
"ports".

Signed-off-by: Christian Eggers <ceggers@arri.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>