dt-bindings: net: fsl,fec: Add nvmem-cells / nvmem-cell-names properties

These properties are inherited from ethernet-controller.yaml.
This fixes the dt_binding_check warning:
imx8mm-tqma8mqml-mba8mx.dt.yaml: ethernet@30be0000: 'nvmem-cell-names',
'nvmem-cells' do not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220720063924.1412799-1-alexander.stein@ew.tq-group.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>