arm: kdump: Add DT properties to crash dump kernel's DTB

Pass the following properties to the crash dump kernel, to provide a
modern DT interface between kexec and the crash dump kernel:

  - linux,elfcorehdr: ELF core header segment, similar to the
    "elfcorehdr=" kernel parameter.
  - linux,usable-memory-range: Usable memory reserved for the crash dump
    kernel.
    This makes the memory reservation explicit, so Linux no longer needs
    to mask the program counter, and rely on the "mem=" kernel parameter
    to obtain the start and size of usable memory.

For backwards compatibility, the "elfcorehdr=" and "mem=" kernel
parameters are still appended to the kernel command line.

Loosely based on the ARM64 version by Akashi Takahiro.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms@verge.net.au>