[TDX] [Kernel] Define the device tree kernel interface for the AP mailbox protocol

[ricardon]

[Migrated from https://github.com/microsoft/HvLite/issues/315]

Need to define where we want to put the device tree discovery for the gpa of the mailbox protocol. The kernel will define the interface that underhill_boot will use to tell the kernel where the interface is for.

[ricardon]

@jyiang5 initially posted on LKML (https://lore.kernel.org/lkml/[email protected]/). The discussion digressed into using the proposed new enable-method for virtual devices. The proposed enable-method is not required to be used only on virtual devices. I can be used on any machine, physical or virtual.

Other architectures use the enable-method as a per-CPU property when describing the system on a device tree.

I will write an updated prototype of the new enable-method and try to get feedback from the maintainers.