Anet A6 LCD Display bright control

LCD Display mod to allow control brightness by GCODE command M355
http://marlinfw.org/docs/gcode/M355.html

A modification is needed on display PCB and recompile Marlin FW.

Mod is based on a Anet A6 display and my https://www.thingiverse.com/thing:2827637, but (I think) it can be done on RepRapDiscount Full Graphic Smart Controller
http://reprap.org/wiki/RepRapDiscount_Full_Graphic_Smart_Controller with standard Smart Adapter (need additional connection to any not used PWM Arduino outpu