tools/qa-checks: Add QA check to validate YAML files

Add yaml_check.py to run yamllint on all yaml files to validate
the yaml files.

Updated yaml files with formatting corrections required to pass
yamllint without errors or warnings on the default settings.

Issue-Id: SCM-3902
Signed-off-by: Joel Goddard <joel.goddard@arm.com>
Change-Id: Ibcd61c5f0a90879724a200d10c5e208c70fdc678