Documentation/process: maintainer-soc: Document purpose of defconfigs

Common mistake in commit messages of patches on mailing list adding
CONFIG options to arm/multi_v7 or arm64/defconfig is saying what that
patch is doing, e.g. "Enable driver foo".  That is obvious from the diff
part, thus explaining it does not bring any value.  What brings value is
to understand why "driver foo" should be in a shared, upstream
defconfig, especially considering that distros have their own defconfigs
and we do not care about non-upstream trees.

Signed-off-by: Krzysztof Kozlowski <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnd Bergmann <[email protected]>

Author: krzk

Documentation/process/maintainer-soc.rst

@@ -207,3 +207,13 @@ The subject line of a pull request should begin with "[GIT PULL]" and made using
 a signed tag, rather than a branch.  This tag should contain a short description
 summarising the changes in the pull request.  For more detail on sending pull
 requests, please see Documentation/maintainer/pull-requests.rst.
+
+Defconfigs purpose
+~~~~~~~~~~~~~~~~~~
+
+Defconfigs are primarily used by the kernel developers, because distros have
+their own configs.  A change adding new CONFIG options to a defconfig should
+explain why the kernel developers in general would want such option, e.g. by
+providing a name of an upstream-supported machine/board using that new option.
+This implies that enabling options in defconfig for non-upstream machines shall
+not be accepted.