platform: Document platform_add_devices() return value

platform_add_devices() returns 0 on success and negative
errno on failure. Document it.

Signed-off-by: Umang Jain <umang.jain@ideasonboard.com>
Link: https://lore.kernel.org/r/20221220085116.19837-1-umang.jain@ideasonboard.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>