Configure and use the PWM output for new BeagleBone AI board

Usage:

   const pwm = require('bbai-pwm');


   pwm.Init('pwm output', period, enable, function(err, stderr) {
       if (err)
       console.log(err);
   });

   pwm output is a string and selects which pwm to use:
       '2a' select and enable pwm output A
       '2b' select and enable pwm output B
       'all' select and enable pwm output A+B

   period in nS
   enable set to 1, enable pwm output
   enable set to 0, disable pwm output


   pwm.DutyCycle('pwm output', dutycycle, function(err, stderr) {
    if (err)
        console.log(err);
   });

   pwm output same as pwm.Init
   dutycycle is the value of the dutycycle between 0 and period

Example:

    const pwm = require('bbai-pwm');

    pwm.Init('2a', 50000, 1, function(err, stderr) {  //period 50000 nanoseconds
    if (err)
        console.log(err);
    });

    pwm.DutyCycle('2a', 25000, function(err, stderr) { // In this example 25000 is half of the output voltage
    if (err)
        console.log(err);
    });

Note: to work you need to enable the pwm devices, in the dtb configuration file