board/BuR/brppt2: fix pwms property warning

Fix the following build-time pwms property warning:

w+arch/arm/dts/imx6dl-brppt2.dtb: Warning (pwms_property):
 /backlight:pwms: property size (12) too small for cell size 3

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>