kho: introduce KHO FDT ABI header

kho: introduce KHO FDT ABI header

Introduce the `include/linux/kho/abi/kexec_handover.h` header file, which
defines the stable ABI for the KHO mechanism.  This header specifies how
preserved data is passed between kernels using an FDT.

The ABI contract includes the FDT structure, node properties, and the
"kho-v1" compatible string.  By centralizing these definitions, this
header serves as the foundational agreement for inter-kernel communication
of preserved states, ensuring forward compatibility and preventing
misinterpretation of data across kexec transitions.

Since the ABI definitions are now centralized in the header files, the
YAML files that previously described the FDT interfaces are redundant.
These redundant files have therefore been removed.

Link: https://lkml.kernel.org/r/20260105165839.285270-5-rppt@kernel.org
Signed-off-by: Jason Miu <jasonmiu@google.com>
Co-developed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Cc: Alexander Graf <graf@amazon.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>