Support SPI

[Peter-van-Tol]

Support SPI for RPi4.

This entails:

• Update configuration, so people can choose which connection the want to make;
• New SPI-driver. Proposed connection string will be spi:<file-descriptor-path>;
• New SPI-bridge for the firmware;

[Peter-van-Tol]

Boilerplate added, the new driver can be added with:

loadrt litexcnc connections="spi:/dev/spidev0.0"

With the boilerplate code, this will load the correct driver and opens a connection with the SPI pins of the card. It will fail the communication to the card, because the communication routines have not been implemented yet. For this a card with the SPI-bridge is required....