dt-bindings: leds: Convert pwm to yaml

The example was adapted in the following ways:

- make use of the now supported 'function' and 'color' properties
- remove pwm nodes, those are documented elsewhere
- align node names to new dt schema rules and dt recommendations

License was not explicitly set before.  The license set now is
recommended by DT project.

Suggested-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Signed-off-by: Alexander Dahl <post@lespocky.de>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Peter Ujfalusi <peter.ujfalusi@ti.com>
[For the license change only:]
Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Pavel Machek <pavel@ucw.cz>