Lower the SPI bus speed to allow split devices.

[M10tech]

I would like to deploy a device on my roof, but cannot get the wifi to connect to indoors.
Considering that 8 wires are enough to connect an E22-900M Lora module to an microcontroller with the wifi,
it would be nice to connect the two halves with like 5m of cat5 cable.
The challenge is that the SPI bus is running at 4MHz which makes this distance a problem.

Idea: allow (by variant config or user config) for the SPI speed to be reduced to say 1MHz or even 500kHz.

Question: is there a bottom limit to the SPI speed at which some things stop working properly?

Observation: some models have SPI speed at 2MHz

And yes, I'm aware of electrical issue as can be found in this excellent article: https://www.ti.com/lit/an/slyt441/slyt441.pdf