You can easily use AMD GPUs on any architecture, as long as you have some PCIe controller, and Linux support for this PCIe controller. AMD GPUs have fully stack open source support in Linux kernel, Mesa 3D drivers (for OpenGL, OpenCL, Vulkan, and other stuff, like video decode/encode), etc. And Intel new dGPUs too. The Nvidia state in Mesa is reasonably good too, and can be definitively be used, but not close to performance of proprietary drivers. However, there are no technical reasons it can’t be supported on other architectures. (I.e. Nvidia GPUs work on x86, POWER and ARM64, and more are possible if there is a will, it is mostly software problem).
Most PCIe controllers are of the shelf IP blocks, that are integrated into the other silicon. This is because PCIe is mostly complex high speed analog / RF signals, and it is not just digital logic. I am pretty sure Mill will be using one of the known IP blocks for this, like Synopsys or Cadence, or such. They are battle tested, certified, have drivers for the host in Linux, comply with PCIe standards, etc. When the time is right it will be there. Mill without PCIe host controllers, would be unusable for anything interesting.