imx8mp-rsb3720-a1: fix pwms property warnings

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>