Initial GPi Case support

This is for the original GPi Case but specifically for the following Pi models:

- Raspberry Pi Zero 2 W
- Raspberry Pi Compute Module 3 via GPi Mate
- Raspberry Pi Compute Module 4 via GPi Mate+

This work in progress as the current issues are know:

- All have no OpenGL hardware acceleration. I think a KMS device tree overlay is required for the screen.
 - CM3 has no sound

Some small suggested README.md changes