mbedTLS: extend days-to-keep

Extend days-to-keep to 60 for mbed-tls-pr-head and mbed-tls-pr-merge
jobs

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I4572d0186232b3429e959a6450014540cf6b6a2f
diff --git a/mbed-tls-pr-head.yaml b/mbed-tls-pr-head.yaml
index 35c23d6..945260c 100644
--- a/mbed-tls-pr-head.yaml
+++ b/mbed-tls-pr-head.yaml
@@ -3,7 +3,7 @@
     description: 'Job for testing mbed tls PR heads'
     project-type: multibranch
     prune-dead-branches: true
-    days-to-keep: 5
+    days-to-keep: 60
     num-to-keep: 1000
     script-path: tests/.jenkins/Jenkinsfile
     properties: