devlink: introduce framework for selftests

Add a framework for running selftests.
Framework exposes devlink commands and test suite(s) to the user
to execute and query the supported tests by the driver.

Below are new entries in devlink_nl_ops
devlink_nl_cmd_selftests_show_doit/dumpit: To query the supported
selftests by the drivers.
devlink_nl_cmd_selftests_run: To execute selftests. Users can
provide a test mask for executing group tests or standalone tests.

Documentation/networking/devlink/ path is already part of MAINTAINERS &
the new files come under this path. Hence no update needed to the
MAINTAINERS

Signed-off-by: Vikas Gupta <vikas.gupta@broadcom.com>
Reviewed-by: Andy Gospodarek <gospo@broadcom.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>