dt-bindings: backlight: Convert common backlight bindings to DT schema

Convert the common GPIO, LED, and PWM backlight bindings to DT schema
format.

Given there's only 2 common properties and the descriptions are slightly
different, I opted to not create a common backlight schema.

Cc: Lee Jones <lee.jones@linaro.org>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Jingoo Han <jingoohan1@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://lore.kernel.org/r/20200630200111.1170742-1-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>