imx8mm-kontron-n801x-s-lvds: fix pwms property warnings

Fix the following build-time pwms property warnings:

w+arch/arm/dts/imx8mm-kontron-n801x-s-lvds.dtb: Warning (pwms_property):
 /backlight:pwms: property size (12) too small for cell size 3

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