dt-bindings: display: novatek, nt36672a: add backlight property

Add backlight property and update example to include it.

Signed-off-by: Joel Selvaraj <jo@jsfamily.in>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/BY5PR02MB70090BB5D8C7D655BEE0642FD9E09@BY5PR02MB7009.namprd02.prod.outlook.com