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

Change-Id: I0dd87adc9e9c958467ed6a7e822f826635b77504
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: James Morse <james.morse@arm.com>
Tested-and-reviewed-by: Bhupesh Sharma <bhsharma@redhat.com>
Git-commit: 3751e728cef2908c15974a5ae44627fd41ef3362
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Akshay Adiga <aadiga@codeaurora.org>