cris: remove arch specific early DT functions

Now that the DT core code handles bootmem arches, we can remove the cris
specific early_init_dt_alloc_memory_arch function. As the default
early_init_dt_add_memory_arch just does a WARN, we can just remove the
entire devicetree.c file.

Cc: Mikael Starvik <starvik@axis.com>
Cc: Jesper Nilsson <jesper.nilsson@axis.com>
Cc: linux-cris-kernel@axis.com
Signed-off-by: Rob Herring <robh@kernel.org>