docs: dt: submitting-patches: add commit subject prefix in reversed format

ASoC, media, regulators and SPI subsystems prefer commits starting with
subsystem name (e.g. "spi: dt-bindings:"), so document this to avoid
confusing contributors.

Cc: Mark Brown <broonie@kernel.org>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240108083750.16350-2-krzysztof.kozlowski@linaro.org


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