dt-bindings: backlight: Add Kinetic KTD253 bindings

This adds device tree bindings for the Kinetic KTD253
white LED backlight driver.

Cc: devicetree@vger.kernel.org
Cc: 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>