ci: Check commit messages for the use of banned words

This commit adds a user defined rule to GitLint.

This new rule takes a list of words, via a CI secret, which should
not be publicly visible in the SDK and checks commit messages to
ensure they do not contain any of these words.

The list of words is provided to GitLint on the commandline using
a configuration option.

Signed-off-by: Anna Bridge <anna.bridge@arm.com>