Use AOSP ATF

Switching to ASOP ATF will make it possible for external users to build
and test Trusty under emulation.

Bug: 134621378
Change-Id: I2042eb1d7dbba0c7942c975603f5519e49115381
diff --git a/default.xml b/default.xml
index e8ab176..77c697b 100644
--- a/default.xml
+++ b/default.xml
@@ -81,6 +81,7 @@
   <project path="external/capstone" name="platform/external/capstone" groups="trusty" />
   <project path="external/dtc" name="platform/external/dtc" groups="trusty" />
   <project path="external/qemu-keycodemapdb" name="trusty/external/qemu-keycodemapdb" revision="10739aa26051a5d49d88132604539d3ed085e72e" groups="trusty" />
+  <project path="external/arm-trusted-firmware" name="trusty/external/trusted-firmware-a" groups="trusty" />
   <!-- END aux external projects -->
 
   <repo-hooks in-project="platform/tools/repohooks" enabled-list="pre-upload" />