CI: allow jobs to be run in merge requests

Out-of-tree users could run an out-of-tree CI with limited coverage,
however it is convenient to be able to run the upstream CI from time
to time. To enable that we would need to change job rules to be able
to run on any GitLab event. Excerpt from GitLab documentation:

> Jobs with no rules default to except: merge_requests

Signed-off-by: Andrejs Cainikovs <andrejs.cainikovs@toradex.com>
Reviewed-by: Tom Rini <trini@konsulko.com>