Feature-gate unavailable peripherals

[hannobraun]

As of #187, Cargo.toml provides a rich set of features that precisely define the hardware used, and can be used to feature-gate all kinds of things. We should at least review which peripherals are available where, and add feature gates accordingly.

I think it would also be advantageous to feature-gate other hardware resources, like GPIO pins and ADC channels. @david-sawatzke expressed some doubt about that.