.travis.yml: Add NRF52_DK to test

By default, Mbed OS PSA is only enabled on K64F and K66F. This
example overrides the configuration to globally enable Mbed OS PSA
as needed.

We build NRF52_DK in Travis to verify this override. This target was
chosen because the pins for atecc608a are already provided in
`mbed_app.json`.

Author: LDong-Arm

.travis.yml

@@ -93,3 +93,15 @@ matrix:
     - <<: *cmake-build-test
       name: "CMake atecc608a example - debug (K64F)"
       env: NAME=cmake_test TARGET_NAME=K64F PROFILE=debug CACHE_NAME=debug-K64F
+
+    - <<: *cmake-build-test
+      name: "CMake atecc608a example - develop (NRF52_DK)"
+      env: NAME=cmake_test TARGET_NAME=NRF52_DK PROFILE=develop CACHE_NAME=develop-NRF52_DK
+
+    - <<: *cmake-build-test
+      name: "CMake atecc608a example - release (NRF52_DK)"
+      env: NAME=cmake_test TARGET_NAME=NRF52_DK PROFILE=release CACHE_NAME=release-NRF52_DK
+
+    - <<: *cmake-build-test
+      name: "CMake atecc608a example - debug (NRF52_DK)"
+      env: NAME=cmake_test TARGET_NAME=NRF52_DK PROFILE=debug CACHE_NAME=debug-NRF52_DK