imx6: aristainetos: fix pwms property warnings

Fix the following build-time pwms property warnings:

w+arch/arm/dts/imx6dl-aristainetos2c_7.dtb: Warning (pwms_property):
 /backlight:pwms: property size (12) too small for cell size 3
w+arch/arm/dts/imx6dl-aristainetos2c_cslb_7.dtb: Warning (pwms_property):
 /backlight:pwms: property size (12) too small for cell size 3

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Heiko Schocher <hs@denx.de>