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>