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 <[email protected]>
Cc: Daniel Thompson <[email protected]>
Cc: Jingoo Han <[email protected]>
Cc: Sam Ravnborg <[email protected]>
Acked-by: Sam Ravnborg <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Rob Herring <[email protected]>
6 files changed