dt-bindings: leds: Convert common LED binding to schema

Convert the common LEDs properties bindings to a schema. As trigger source
providers are different nodes, we need to split trigger source properties
to a separate file.

Bindings for LED controllers can reference the common schema for the LED
child nodes:

patternProperties:
  "^led@[0-4]":
    type: object
    allOf:
      - $ref: common.yaml#

Cc: Pavel Machek <pavel@ucw.cz>
Cc: Dan Murphy <dmurphy@ti.com>
Cc: linux-leds@vger.kernel.org
Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>