of/fdt: consolidate built-in dtb section variables

Unify the various architectures __dtb_start and __dtb_end definitions
moving them into of_fdt.h.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Vineet Gupta <vgupta@synopsys.com>
Acked-by: James Hogan <james.hogan@imgtec.com>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: linux-metag@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: linux@lists.openrisc.net
Cc: linux-xtensa@linux-xtensa.org
Tested-by: Grant Likely <grant.likely@linaro.org>
Tested-by: Stephen Chivers <schivers@csc.com>