Enable Beaglebone Black LCD3 backlight dimming

To enable backlight dimming using the CircuitCo LCD3 cape, you need to connect the backlight boost circuit EN line to EHRPWM1A, by removing R126 and adding a 0R resistor (or blob of solder) to the pads for R123.

Then, you can issue the command

echo 38 > /sys/class/backlight/backlight/brightness

to control backlight intensity.