Enable A7 for PWM

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 hardware setup that is was in fact supported by the hardware.

I will be making an associated PR against https://github.com/rwaldron/spark-io with the required change to relax the validation on that side.