arm64: apple: Add initial Apple Mac mini (M1, 2020) devicetree

This currently supports:

* SMP (via spin-tables)
* AIC IRQs
* Serial (with earlycon)
* Framebuffer

A number of properties are dynamic, and based on system firmware
decisions that vary from version to version. These are expected
to be filled in by the loader.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hector Martin <marcan@marcan.st>