doc: update faq.md

Make the faq.md a bit more up to date:
 - Update links
 - Update Table of Contents
 - Refer to Security Advisories page
 - Mention Travis OP-TEE build.git job

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Igor Opaniuk <igor.opaniuk@linaro.org>