dt-bindings: leds: Add LED_FUNCTION definitions

Add initial set of common LED function definitions.

Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Cc: Baolin Wang <baolin.wang@linaro.org>
Cc: Daniel Mack <daniel@zonque.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Oleh Kravchenko <oleg@kaa.org.ua>
Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: Simon Shields <simon@lineageos.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Dan Murphy <dmurphy@ti.com>