dt-bindings: backlight: Add lm3630a bindings

Add new backlight bindings for the TI LM3630A dual-string white LED.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>