pwm.md: Added information on how to turn on a display by directly talking to the PWM chip

While unsing the SysFS driver for the PWM chip works, time has shown that some times for no reason the display or the background light of the display would not turn on when using the pwmchipXY Linux device. A better way seems to be to talk to the PWM chip directly and forego the SysFS option. For now we leave the SysFS info in the docs, if someone wants to go this way.