[FEATURE] Feature request - Port to Raspberry Pi 4

[bSchnepp]

Issue Checklist

• A related or similar issue is not already marked as open
• Another issue describing a similar feature has not already been marked as wontfix or closed
• This feature is not already present in the software

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Feature Description
A port to the Raspberry Pi 4 should be done. This will allow pantheon to boot on a inexpensive arm64 board, which is (relatively) widely available and popular.

Feature Benefits
List the reasons why this feature would be beneficial

• The Pi is a very popular device, of which I already have some to test with. (B0 stepping, however) The B0 stepping specifically has some problems (3GB PCIe window limitation, emmc within the first 1G, which is precious and should be used for GPU device code mappings..., we should be extra careful since C0 is different enough to mess up uboot, etc.)
• A Mesa driver already exists: we just need to wrap DRM/DRI/KMS/etc. as userland daemons (or even within one big 'gpu' process) and should be good to go. That's a ton of work, but much easier than reimplementing DRM/KMS from scratch. Or we can reimplement DRM from scratch anyway, and have a useful reference driver.
• We can see this work on the real thing!

Use case examples
List examples where this feature could be useful for end users.

• Deployment on pantheon in small CM4 devices (ie, IoT stuff)
• Deployment of pantheon for normal Pi 4 Model B, as a reasonable alternative to Linux

Additional information
Any additional information should be placed here.