Fix the following build-time pwms property warnings:
w+arch/arm/dts/imx8mp-rsb3720-a1.dtb: Warning (pwms_property):
/lvds_backlight@0:pwms: property size (12) too small for cell size 3
w+arch/arm/dts/imx8mp-rsb3720-a1.dtb: Warning (pwms_property):
/lvds_backlight@1:pwms: property size (12) too small for cell size 3
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>