arm: dts: mt2701: add pwm backlight device node

This patch adds the device node for MT2701 pwm backlight.

Signed-off-by: Weiqing Kong <weiqing.kong@mediatek.com>
Signed-off-by: Erin Lo <erin.lo@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>