dt-bindings: backlight: Add Kinetic KTD259 bindings

The KTD259 is just a newer version of the KTD253 and works
all the same.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>