Documentation/process: maintainer-soc: add clean platforms profile

Some SoC platforms require that commits must not bring any new
dtbs_check warnings. Maintainers of such platforms usually have some
automation set, so any new warning will be spotted sooner or later.
Worst case: they run the tests themselves. Document requirements for
such platforms, so contributors can expect their patches being dropped
or ignored, if they bring new warnings for existing boards.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230723131924.78190-2-krzysztof.kozlowski@linaro.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>