arm64: dts: rockchip: Add nodes for NPU and its MMU to rk3588s

See Chapter 36 "RKNN" from the RK3588 TRM (Part 1).

This is a derivative of NVIDIA's NVDLA, but with its own front-end
processor.

The IP is divided in three cores, programmed independently. The first
core though is special, requiring to be powered on before any of the
others can be used.

The IOMMU of the first core is also special in that it has two subunits
(read/write?) that need to be programmed in sync.

v2:
- Have one device for each NPU core (Sebastian Reichel)
- Have one device for each IOMMU (Sebastian Reichel)
- Correctly sort nodes (Diederik de Haas)
- Add rockchip,iommu compatible to IOMMU nodes (Sebastian Reichel)

Signed-off-by: Tomeu Vizoso <tomeu@tomeuvizoso.net>