arm64: kexec_file: add crash dump support

Enabling crash dump (kdump) includes
* prepare contents of ELF header of a core dump file, /proc/vmcore,
  using crash_prepare_elf64_headers(), and
* add two device tree properties, "linux,usable-memory-range" and
  "linux,elfcorehdr", which represent respectively a memory range
  to be used by crash dump kernel and the header's location

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Reviewed-by: James Morse <james.morse@arm.com>
Tested-and-reviewed-by: Bhupesh Sharma <bhsharma@redhat.com>
Signed-off-by: Will Deacon <will@kernel.org>