kexec: Move ELF fields to struct kimage

ELF related fields elf_headers, elf_headers_sz, and elf_load_addr are
defined in architecture specific 'struct kimage_arch' for x86, powerpc,
and arm64.  The name of these fields are different in these
architectures that makes it hard to have a common code for setting up
the device tree for kexec system call.

Move the ELF fields to 'struct kimage' defined in include/linux/kexec.h
so common code can use it.

Suggested-by: Rob Herring <robh@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
Reviewed-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210221174930.27324-2-nramas@linux.microsoft.com