Fixed some issues and made some changes suggested by the mailing list:

* Split the patch into 2
* Add a file to the Documentation area for pwm-sunxi
* Polarity was fixed at 0 (active low) - this was a major bug
* Fixed the 0% and 100% to behave as expected
* Lightened up the code executed when *just* the period or duty is changed so that
  the other pwm channel isn't adversely affected and the period / duty switches
  as quickly as possible
* Added some more error checking - return EINVAL when invalid input is given