dt-bindings: backlight: Add some common backlight properties

Let's use a common.yaml include for the backlight like we do with
the LEDs. The LEDs are inherently incompatible so their bindings
cannot be reused for backlight.

Cc: devicetree@vger.kernel.org
Suggested-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>