docs: dt: writing-bindings: Update URL to DT schemas

The previous URL was giving a 404 error.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220627223950.35748-1-paul@crapouillou.net