metag: remove arch specific early DT functions

Now that the DT core code handles bootmem arches, we can remove the metag
specific early_init_dt_alloc_memory_arch function. As the default
early_init_dt_add_memory_arch just does a WARN, we can remove it too.

Cc: James Hogan <jhogan@kernel.org>
Cc: linux-metag@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>