i2creg, onewirereg, spireg: augment Ref to include the pins

[maruel]

Ported from google/periph#257.

The goal is to be able to list the pin used by each I²C/1-wire/SPI port/bus without having to open the port/bus, as this is currently the case.

To achieve this, the corresponding Ref needs to be augmented with the corresponding pin names (RX/TX/CS/etc). This in itself isn't so bad and could be not considered a breaking change.

The breaking change is that this information needs to be passed to the Register() function, otherwise the register function would have to open the port/bus itself, which would slow down registration and would potentially cause unintended side effects. So if the Register() function needs to be changed, this is a breaking change.

Ref:

• https://periph.io/x/conn/v3/spi/spireg#Ref
• https://periph.io/x/conn/v3/i2c/i2creg#Ref
• https://periph.io/x/conn/v3/onewire/onewirereg#Ref