dt-bindings: vendor-prefixes: document deprecated Atheros

Two old boards use "atheros" prefix instead of already documented "qca".
Document it as deprecated to fix warnings like:

  at91-gatwick.dtb: atheros@0: 'atheros,board-id' does not match any of the regexes

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220601135222.205035-1-krzysztof.kozlowski@linaro.org