Add Coverity Scan support

I've recently signed up dtc for Coverity Scan coverage.  This adds magic
to the .travis.yml file to submit builds to Coverity for analysis.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

.travis.yml

@@ -1,5 +1,11 @@
 language: c
 
+# Coverity Scan uploads
+env:
+  global:
+  # COVERITY_SCAN_TOKEN (dgibson/dtc)
+  - secure: "vlHvXe618//IM9LQaKzqsrUbjs7ng0L9UCST4kJbJnFQDXvVe5JiSmJGd4ef7mm0NUv5bMRl2W3xCiu6BYAu/NvU3tMNHoLG+JgCJs0+wLJXbWOwji/NmH7olqgJG+CmpaCMXjARF6+nrTnBYHJL6cYyf4KVoV4B0I/hLUW91+s="
+
 matrix:
   include:
     - addons:
@@ -7,6 +13,14 @@ matrix:
           packages:
             - swig
             - python-dev
+        coverity_scan:
+          project:
+            name: dtc
+            description: Device Tree Compiler
+          notification_email: david@gibson.dropbear.id.au
+          build_command: make
+          branch_pattern: coverity_scan
+
     - addons:
         apt:
           packages: