doc: Update link URLs to trustedfirmware.org repos

trustedfirmware.org (tf.org) git browse switched from cgit to
gitiles. Update URLs in links to tf.org project repos:
- Fix the broken link URL to Trusted Service repo.
- Update the link URL to TF-A repo in case that its default branch
  changes later.
- Update the link URL to TF-M repo in case that it later removes
  deprecated master branch.

Signed-off-by: David Hu <david.hu2@arm.com>