Documentation/process: maintainer-soc: document dtbs_check requirement for Samsung

Samsung ARM/ARM64 SoCs (except legacy S5PV210) are also expected not to
bring any new dtbs_check warnings. In fact this have been already
enforced and tested since few release.

Cc: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230712084131.127982-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/process: add soc maintainer handbook

Arnd suggested that adding a maintainer handbook for the SoC "subsystem"
would be helpful in trying to bring on board maintainers for the various
new platforms cropping up in RISC-V land.

Add a document briefly describing the role of the SoC subsystem and some
basic advice for (new) platform maintainers.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>