metag: move setup_machine_fdt declaration from prom.h

Move setup_machine_fdt out of prom.h and into asm/setup.h in preparation
to remove prom.h.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: linux-metag@vger.kernel.org