mbedtls-amis: Add triggers

Trigger the job by the change-merged-event event of next/ci/mbedtls-amis
repo

Change-Id: I7c9843e0a343d355b301811a762ca25b4cd2154a
diff --git a/mbedtls-amis.yaml b/mbedtls-amis.yaml
index da4f2f7..e6d4f6f 100644
--- a/mbedtls-amis.yaml
+++ b/mbedtls-amis.yaml
@@ -14,6 +14,18 @@
     node: build-amd64-private
     concurrent: false
     display-name: 'build AMIs for mbedtls'
+    triggers:
+        - gerrit:
+            server-name: 'review.trustedfirmware.org'
+            trigger-on:
+                - change-merged-event
+            projects:
+                - project-compare-type: 'PLAIN'
+                  project-pattern: 'next/ci/mbedtls-amis'
+                  branches:
+                    - branch-compare-type: 'PLAIN'
+                      branch-pattern: 'master'
+            silent-start: true
     builders:
         - shell:
             !include-raw: mbedtls-amis/builders.sh