No udev events when exporting / unexporting pwm devices on BeagleBone Blue

On kernel 4.4.49-ti-r89 of the default beaglebone blue image, when exporting and un-exporting there is no udev event fired.

```
root@beaglebone:/sys/devices/platform/ocp/48302000.epwmss/48302200.pwm/pwm/pwmchip2# echo 0 > unexport
root@beaglebone:/sys/devices/platform/ocp/48302000.epwmss/48302200.pwm/pwm/pwmchip2# echo 0 > export
```

No udev events from ` udevadm monitor`

With the gpio lines you get a udev event for export/uneport:

```
root@beaglebone:/sys/class/gpio# echo 10 > unexport
root@beaglebone:/sys/class/gpio# echo 10 > export
```
generates:

```
debian@beaglebone:~$ udevadm monitor
monitor will print the received events for:
UDEV - the event which udev sends out after rule processing
KERNEL - the kernel uevent

KERNEL[449.118706] remove /devices/platform/ocp/44e07000.gpio/gpio/gpio10 (gpio)
UDEV [449.193955] remove /devices/platform/ocp/44e07000.gpio/gpio/gpio10 (gpio)
KERNEL[454.849166] add /devices/platform/ocp/44e07000.gpio/gpio/gpio10 (gpio)
UDEV [456.275388] add /devices/platform/ocp/44e07000.gpio/gpio/gpio10 (gpio)
```

With no udev rules to firing it makes it impossible to have a rule to set the permissions. This is compounded by the having to unexport the export one channel to set the frequency of the pwm chip.