dt-bindings: rng: Add vmgenid support

Virtual Machine Generation ID driver was introduced in commit
af6b54e2 ("virt: vmgenid: notify RNG of VM fork and supply
generation ID"), as an ACPI only device.

VMGenID specification http://go.microsoft.com/fwlink/?LinkId=260709


defines a mechanism for the BIOS/hypervisors to communicate to the
virtual machine that it is executed with a different configuration (e.g.
snapshot execution or creation from a template).  The guest operating
system can use the notification for various purposes such as
re-initializing its random number generator etc.

As per the specs, hypervisor should provide a globally unique
identified, or GUID via ACPI.

This patch tries to mimic the mechanism to provide the same
functionality which is for a hypervisor/BIOS to notify the virtual
machine when it is executed with a different configuration.

As part of this support the devicetree bindings requires the hypervisors
or BIOS to provide a memory address which holds the GUID and an IRQ
which is used to notify when there is a change in the GUID.  The memory
exposed in the DT should follow the rules defined in the vmgenid spec
mentioned above.

Reason for this change: Chosing ACPI or devicetree is an intrinsic part
of an hypervisor design.  Without going into details of why a hypervisor
would chose DT over ACPI, we would like to highlight that the
hypervisors that have chose devicetree and now want to make use of the
vmgenid functionality cannot do so today because vmgenid is an ACPI only
device.  This forces these hypervisors to change their design which
could have undesirable impacts on their use-cases, test-scenarios etc.

The point of vmgenid is to provide a mechanism to discover a GUID when
the execution state of a virtual machine changes and the simplest way to
do it is pass a memory location and an interrupt via devicetree.  It
would complicate things unnecessarily if instead of using devicetree, we
try to implement a new protocol or modify other protocols to somehow
provide the same functionility.

We believe that adding a devicetree binding for vmgenid is a simpler,
better alternative to provide the same functionality and will allow such
hypervisors as mentioned above to continue using devicetree.

More references to the vmgenid specs are found below.

Signed-off-by: Sudan Landge <sudanl@amazon.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Alexander Graf <graf@amazon.com>
Link: https://www.qemu.org/docs/master/specs/vmgenid.html
Link: https://learn.microsoft.com/en-us/windows/win32/hyperv_v2/virtual-machine-generation-identifier


Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>