Reworked the utility/rp2 SPI class and removed static members

[TheGarkine]

In the discussion in #891, we came to the conclusion, that the members in the rp2 utility SPI class should not be static, since it is otherwise not possible to connect two (or more) radios on multiple SPI busses.

I refactored this behavior and added an example of how to set up multiple radios on different SPI buses.

[2bndy5]

I'm not sure we need a new example to demo this. I'd actually prefer to have the necessary code as an example snippet in the /docs/pico_sdk.md at the bottom in a new section titled "## Multi-Radio Scenario". My reason here is because the examples use examples_pico/defaultPins.h to declare the CE, CSN, and IRQ pins used for a single radio. I use defaultPins.h to build the Pico SDK examples for multiple RP2040 boards (some of which don't have enough pins exposed for 2 radios).

I can force push this change if you want (not sure if you're comfortable with force pushed commits). You do seem very proficient in English writing though (👍🏼).

[2bndy5]

I moved the example into the pico_sdk doc because it wasn't actually doing anything with the radio objects.

[TheGarkine]

Sorry, I was a bit busy on the weekend. Yeah, that's totally understandable reasoning, and thank you for your initiative! Do you need any other changes in order to move forward with this PR?

[2bndy5]

No, everything LGTM.

[2bndy5]

@XDeschuyteneer I moved your comment to a separate thread #912. Please continue your inquiry there.