dt-bindings: net: snps,dwmac: Document queue config subnodes

The queue configuration is referenced by snps,mtl-rx-config and
snps,mtl-tx-config. Some in-tree DTs and the example put the
referenced config nodes directly beneath the root node, but
most in-tree DTs put it as child node of the dwmac node.

This adds proper description for this setup, which has the
advantage of validating the queue configuration node content.

The example is also updated to use the sub-node style, incl.
the axi bus configuration node, which got the same treatment
as the queues config in 5361660a ("dt-bindings: net: snps,dwmac:
Document stmmac-axi-config subnode").
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>