powerpc: Use ELF fields defined in 'struct kimage'

ELF related fields elf_headers, elf_headers_sz, and elfcorehdr_addr
have been moved from 'struct kimage_arch' to 'struct kimage' as
elf_headers, elf_headers_sz, and elf_load_addr respectively.

Use the ELF fields defined in 'struct kimage'.

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/20210221174930.27324-4-nramas@linux.microsoft.com