dt-bindings: net: ethernet-controller: add 2.5G and 10G speeds

Both are already used by HW and drivers inside Linux.

Fix warnings as:
arch/arm64/boot/dts/freescale/fsl-ls1028a-kontron-sl28-var2.dt.yaml: ethernet@0,2: fixed-link:speed:0:0: 2500 is not one of [10, 100, 1000]
        From schema: Documentation/devicetree/bindings/net/ethernet-controller.yaml

Signed-off-by: David Heidelberg <david@ixit.cz>
Link: https://lore.kernel.org/r/20211124202046.81136-1-david@ixit.cz


Signed-off-by: Rob Herring <robh@kernel.org>