Add support for Tenstorrent Atlantis

[remexre]

A patch series to add support for this to QEMU got submitted last week (updated one submitted today); I'm probably going to try and get one as soon as they're available.

This would be the first chip we're running on with the H extension for hardware virtualization support, so anyone interested in virtualization might be interested in this one.

It's also the fastest CPU we'd be able to easily get access to.

Given that it has upstream Linux and OpenSBI support, this hopefully should be really easy for us to bring up; its UART is ns16550a-compatible, and it generally looks very spec-compliant.

It probably makes sense to hold off on starting this until a QEMU release with it gets in, but this is for tracking for now.

From the QEMU patch series:

Tenstorrent Atlantis (tt-atlantis)

The Tenstorrent Atlantis platform is a collaboration between Tenstorrent
and CoreLab Technology. It is based on the Atlantis SoC, which includes
the Ascalon-X CPU and other IP from Tenstorrent and CoreLab Technology.

The Tenstorrent Ascalon-X is a high performance 64-bit RVA23 compliant
RISC-V CPU.

Features

• 8-core Ascalon-X CPU Cluster
• Dual x32 LPDDR5 @ 6400 MT/s
• RISC-V compliant Advanced Interrupt Architecture
• PCIe Gen4
• RISC-V compliant IOMMU
• GPU and Video subsystem
• 2x USB3.1 & 2x USB2.0
• 2x 1GbE Ethernet
• 2x eMMC5.1/SDIO3.0 storage
• Extensive connectivity (SPI, I2C, UART, GPIO, CANFD)

Note: the QEMU tt-atlantis machine does not model the platform
exactly or all devices, but it is undergoing improvement.

Supported software

The Tenstorrent Ascalon CPUs avoid proprietary or non-standard
extensions, so compatibility with existing software is generally
good. The QEMU tt-atlantis machine works with upstream OpenSBI
and Linux with default configurations.

The development board hardware will require some implementation
specific setup in firmware which is being developed and may
become a requirement or option for the tt-atlantis machine.