dt-bindings: net: dsa: lantiq,gswip: convert to YAML schema

dt-bindings: net: dsa: lantiq,gswip: convert to YAML schema

Convert the lantiq,gswip bindings to YAML format.

Also add this new file to the MAINTAINERS file.

Furthermore, the CPU port has to specify a phy-mode and either a phy or
a fixed-link. Since GSWIP is connected using a SoC internal protocol
there's no PHY involved. Add phy-mode = "internal" and a fixed-link to
the example code to describe the communication between the PMAC
(Ethernet controller) and GSWIP switch.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20240611135434.3180973-2-ms@dev.tdt.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>