arm64: dts: imx8mq-librem5-r4: Allow for higher backlight current

As per datasheet the LED supply current can be up to 25mA. Eric tested
this for longer period of time.
2 jobs for evg-led in 60 minutes and 37 seconds (queued for 1 second)
latest
Status Name Job ID Coverage
  Build
passed build:aarch64 #314725
librem5

00:18:25

passed build:deb #314726
librem5:arm64

01:00:36