net: dt-bindings: ixp4xx-ethernet: Support fixed links

This ethernet controller is using fixed links for DSA switches
in two already existing device trees, so make sure the checker
does not complain like this:

intel-ixp42x-linksys-wrv54g.dtb: ethernet@c8009000 (intel,ixp4xx-ethernet):
'fixed-link' does not match any of the regexes: '^pinctrl-[0-9]+$'
from schema $id: http://devicetree.org/schemas/net/intel,ixp4xx-ethernet.yaml#

intel-ixp42x-usrobotics-usr8200.dtb: ethernet@c800a000 (intel,ixp4xx-ethernet):
'fixed-link' does not match any of the regexes: '^pinctrl-[0-9]+$'
from schema $id: http://devicetree.org/schemas/net/intel,ixp4xx-ethernet.yaml#



Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202507040609.K9KytWBA-lkp@intel.com/


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20250704-ixp4xx-ethernet-binding-fix-v1-1-8ac360d5bc9b@linaro.org


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