ARM i.MX53: mba53: Fix PWM backlight DT node

The i.MX53 PWM controller uses two cells to describe the PWM specifier.
Remove the extra unused values from the backlight DT node pwms property.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>