dt-bindings: leds: Add regulator-led binding

The regulator is a LED connected directly to a regulator and with
its brightness controlled by the voltage of the regulator.

Cc: devicetree@vger.kernel.org
Cc: Antonio Ospite <ao2@ao2.it>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>