github: Add dependabot configuration

Add a GH dependabot configuration to check for updates for github
actions.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>
Co-authored-by: Benjamin Cabé <benjamin@zephyrproject.org>

.github/dependabot.yml

@@ -0,0 +1,13 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    commit-message:
+      prefix: "ci: github: "
+    labels: []
+    groups:
+      actions-deps:
+        patterns:
+          - "*"