Raspberry Pi 4B SPI driver

[Matteo Golin <ma...@gmail.com>]

Hello everyone!

I've opened a PR for an SPI driver for the BCM2711 (Raspberry Pi 4B chip) here: https://github.com/apache/nuttx/pull/16428

I would appreciate any review, especially if someone has a Pi 4B and a breadboard SPI device they can try this out on.
Unfortunately it appears only the SPI0 interface is initialized by the Raspberry Pi provided firmware/bootloader, so I
haven't been able to get any results on the remaining interfaces. The BCM2711 documentation is quite limited and I would
need to spend some time doing reverse-engineering on the Linux drivers or searching the web more than I already have. On
the off-chance that anyone here has written bare-metal programs for the Pi 4B or knows about firmware initialization for
the BCM2711, I would appreciate any help I can get in understanding how to set up the peripherals better!

Best,

-- 
Matteo Golin