Enable PWM on pin A7 since it does work.

Not only does it work , but also it is needed for using the Spark ["Shield Shield"](https://github.com/spark/docs/blob/master/docs/shields.md#shield-shield) with a L298P motor shield with the default jumper configuration.

I knew that Spark Core supported, due to testing their RC car sketch. After some further experimentation, I determined by testing my physical setup that it was in fact supported by the hardware.

I have already made an associated PR https://github.com/voodootikigod/voodoospark/pull/18 with the required change to allow within VoodooSpark side.