busio.SPI: specify max frequency and desired frequency in constructor ?

[dhalbert]

Some chips have SPI peripherals that can vary in max frequency. For instance, the nRF52840 has 8MHz max normally, but has one that can do 32Mhz. Currently we have no way to specify which we would prefer at construction.

It would also be nice to specify the starting frequency in the constructor, rather than having to call configure(). The other configure() args could also be passed in, but that's less important.

Also, I'm not sure why it's called baudrate instead of frequency for SPI. We could change it, though maybe it's not worth it.

[dhalbert]

I started doing this when doing #2613, by adding a desired_frequency argument to the constructor. But there are other considerations about picking a SPI peripheral, like whether it is shared with I2C, and realized it was hard to specify all of those in the constructor. So I documented that the "best" SPI is picked first (highest frequency), and they are picked in decreasing order of desirability (like non-shared ones picked after that).