TF-A: Update time days-to-keep

This is set too high, currently is eating up quite a lot of diskspace
(over 1.5T currently). Lets bring down to a more sane number.

Signed-off-by: Benjamin Copeland <ben.copeland@linaro.org>
Change-Id: I20a893093a03f8a4190a0b972987045a2633b0d2
diff --git a/tf-a-builder.yaml b/tf-a-builder.yaml
index b9017a9..239eaf6 100644
--- a/tf-a-builder.yaml
+++ b/tf-a-builder.yaml
@@ -9,7 +9,7 @@
       Trusted Firmware A (TF-A) builder
     properties:
         - build-discarder:
-            days-to-keep: 45
+            days-to-keep: 15
         - authorization:
             !include: authorization.yaml.inc
     parameters: