Phasing out most Linux drivers

[2bndy5]

I'm of the opinion that all but SPIDEV should be phased out.

• MRAA isn't well maintained since Intel passed it on to the eclipse github org; contributions don't seem to be well verified/tested. And it stopped working on the RPi4.
• BCM2835 can't really support RPi5 using interactions with CPU registers due to the hardware abstraction on RPi5.
• pigpio is practically in the same position as BCM2835 lib concerning RPi5 support. They'd have to overhaul the lib's internals to behave more like the newer lgpio lib of the same author, but that would introduce breaking changes to the API.
• wiringPi has been limping on via sparse community support since Gordon ended official development. The more I dive into that lib's source, the less appealing it is. Long story made short: its old and not well written.
• Has littlewire ever worked? The source I found is not compatible with the driver API we have. The header that our littlewire driver looks for no longer exists in the littlewire sources, and I can't even figure out when that changed using git blame.

Originally posted by @2bndy5 in #969 (comment)

[2bndy5]

I don't expect to actually remove anything until RPi7 gets produced. We can hard-code the build systems to select SPIDEV despite the presence of other needed libraries. This should be the least breaking approach we can do to dissuade users from selecting other (seemingly unreliable) drivers.

[marksolly]

I see you haven't had a lot of feedback on this yet. I have just stumbled into this library today specifically because of the BCM2835 driver support and needing ultra low latency access to an nRF24 to achieve 160us round trip times from reading RX FIFO to writing to the TX FIFO. This has proven impossible with spidev/ioctl calls. It would be a shame to see direct BCM2835 support dropped while any hardware that supports it remains available for purchase.