dt-bindings: net: Convert fsl,gianfar to YAML

Add a binding for the "Gianfar" ethernet controller, also known as
TSEC/eTSEC.

Signed-off-by: J. Neuschäfer <j.ne@posteo.net>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20250228-gianfar-yaml-v2-3-6beeefbd4818@posteo.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>