docs: dt: writing-schema: mention yamllint

The dtschema uses yamllint, if present, to check the syntax of the
files, so document this.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220419084942.27409-1-krzysztof.kozlowski@linaro.org