Created fake PWM device for HIL

[LorenzMeier]

This creates a fake PWM device for hardware in the loop setups. This is needed to be able to load a mixer a la:

fmu mode_hil_pwm
mixer load /dev/pwm_output /etc/mixers/FMU_AET.mix

This is not an actual pull request, but rather a convenient way to send a diff via web. There is still a missing piece around the lines below - without a call to up_pwm_ioctl() the mixer file is not loaded - can this safely just be added to the HIL 8 PWM case?

switch(_mode) {
    case MODE_2PWM:
    case MODE_4PWM:
        ret = pwm_ioctl(filp, cmd, arg);
        break;
    case MODE_HIL_8PWM:
        /* do nothing */
        debug("loading mixer for virtual HIL device");
        ret = 0;
        break;
    default:
        debug("not in a PWM mode");
        break;
}

[px4dev]

If you want a fake 8pwm mode, and you want to implement it with the fmu driver, then you'll need to do it properly.

However, it would almost certainly be simpler and cleaner to have a separate HIL PWM output driver.