This commit introduces the following targets:

- `format-yaml`: automatically formats any YAML files
- `check-yaml`: checks the formatting on YAML files
- `lint-yaml`: lints YAML files

These are executed implicitly as part of the `format`, `check` and
`lint` targets.

yamllint can be found at: https://yamllint.readthedocs.io/en/stable



The Python-based formatting script introduced in this commit is
relatively simple, supporting several options useful for both a
continuous integration and developer environment, and supports both
automatic formatting of YAML source files as well as generation of
unified diffs.

Change-Id: I6b5b2d8609e7797589aa524a74cf1a6ccfc4eebe
Signed-off-by: Chris Kay <chris.kay@arm.com>
Signed-off-by: Joel Goddard <joel.goddard@arm.com>
Signed-off-by: Girish Pathak <girish.pathak@arm.com>