github: dependabot: Add configuration

Add a dependabot configuration that's aligned with Zephyr upstream. This is
particularly important now that we use pinned action versions so we get
notified when they need updating.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>

Author: jhedberg

.github/dependabot.yml

@@ -0,0 +1,16 @@
+version: 2
+enable-beta-ecosystems: true
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    cooldown:
+      default-days: 7
+    commit-message:
+      prefix: "ci: github: "
+    labels: []
+    groups:
+      actions-deps:
+        patterns:
+          - "*"