[source] buttons on docs.rs do not work

[jonas-schievink]

https://docs.rs/nrf52840-hal/0.15.1/nrf52840_hal/struct.Timer.html

Clicking on any of the [source] buttons here results in a 404.

This happens because the source code is in the nrf_hal_common crate, which does not get properly built on docs.rs: right now it tries to build without enabling a chip feature.

The short-term fix for some chips only is to tell docs.rs to enable one of the chip features (preferably one with all the peripherals). The long-term fix probably involves changing how the HAL is structured.