travis/gitlab/azure: Run cppcheck in parallel

This takes ages to run single-threaded. Adjust it to use all available
processors.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/.travis.yml b/.travis.yml
index bd2ac4e..b3253da 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -448,7 +448,7 @@
     # static code analysis with cppcheck (we can add --enable=all later)
     - name: "cppcheck"
       script:
-        - cppcheck --force --quiet --inline-suppr .
+        - cppcheck -j$(nproc) --force --quiet --inline-suppr .
     # build HTML documentation
     - name: "htmldocs"
       script: