openrisc: Refactor or32_early_setup()

  - Change fdt pointer (passed from head.S) from unsigned int to void *,
    which allows to kill a cast, and makes it compatible with __dtb_start.
  - Use pr_info(),
  - Extract common part.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux@lists.openrisc.net
Signed-off-by: Jonas Bonn <jonas@southpole.se>