dt-bindings: backlight: lm3630a: Add enable-gpios to describe HWEN pin

Provide DT documentation for enable-gpios.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>