docs: mtd: spi-nor: add sections about flash additions and testing

docs: mtd: spi-nor: add sections about flash additions and testing

Add sections about how to propose a new flash addition and about the
minimum testing requirements.

Reviewed-by: Michael Walle <michael@walle.cc>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Link: https://lore.kernel.org/r/20231124184902.1194235-2-tudor.ambarus@linaro.org
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>