Fix validate-all-yaml.sh to check for files with yml extension

YAML files can have .yaml or .yml extension. Make `validate-all-yaml.sh`
check for *.yml files in addition to *.yaml files since we now have some
of these in our source tree.

This should help to prevent bogus YAML files to land in the repository.

Change-Id: Id59273680fc2424f7b636033b1d641efcaaeba22