feat(dependabot): use also for git submodules

- at the time of writing we have one git submodule
  9elements/firmware-action-example which is used for documentation
  purposes and should be kept up-to-date

Signed-off-by: AtomicFS <vojtech.vesely@9elements.com>

.github/dependabot.yml

@@ -44,3 +44,9 @@ updates:
         update-types:
           - 'minor'
           - 'patch'
+
+  - package-ecosystem: "gitsubmodule"
+    directory: "/"
+    schedule:
+      interval: "daily"
+      time: "00:00"