MLK-18042-02 arm: dts: update the backlight brightness on imx7ulp evk

MLK-18042-02 arm: dts: update the backlight brightness on imx7ulp evk

On the i.MX7ULP EVK Rev.B baord, the backlight brigntness driver circuit
is updated. A RC filter is added on the MP3301's EN pin. So the PWM's frequency
should be change to 20KHZ. for EN pin, A DC voltage from 0.7V to 1.4V can control
the LED current from 0% to 100%. the backlight brightness level also need to be
updated.

Signed-off-by: Bai Ping <ping.bai@nxp.com>
Reviewed-by: Anson Huang <Anson.Huang@nxp.com>
(cherry picked from commit 82555e15a5f958c09492d0103425dc30bc7cd927)