ARM: dts: qcom: msm8974-hammerhead: add support for backlight

Add necessary device tree nodes for the main LCD backlight.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>