gasket: add a framework for Google accelerator ASICs

[guest-seuros]

Summary

Gasket is Google's name for the layer their accelerator drivers share:
Google ASIC Software, Kernel Extensions, and Tools. The chips differ,
but they present the same shape to the host. A control register BAR
with a few windows user space may map, an on-chip page table the host
fills in so the device can reach host memory, and a block of MSI-X
vectors.

Test Plan

amd64 only for now, and host mappings are refused under KMSAN.
only the m2 cards were tested.

[guest-seuros]

fix docs