Skip external dependencies in coverity checks

Before, The coverity scan would pick up defects within external
dependencies. This introduces many false positives, as these external
dependencies don't necessarily follow the same conventions as TF-A. This
improves the coverity filter to remove these events from our totals.

Change-Id: I029cb6e9d2b45845be07789856765c729d97cb3b
1 file changed