Add tf-a-lts-patch-stack-watcher.yaml

This job will trigger Allow-CI job, Allow-CI+2, by default,
if a new patch/patch stack is submitted to the lts branches.
It also monitors the existing unmerged patch/patch stack,
if the submit requirement is met, it will merge it. [1]

[1]: https://linaro.atlassian.net/browse/TFC-546

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I59e62182573ae4ef67989ccb119806890a30da6c
2 files changed