ralink: Introduce fw_passed_dtb to arch/mips/ralink

ralink: Introduce fw_passed_dtb to arch/mips/ralink

This patch adds fw_passed_dtb to arch/mips/ralink to support
CONFIG_MIPS_RAW_APPENDED_DTB. Furthermore it adds a check that __dtb_start is
not the same address as __dtb_end.

Signed-off-by: Tobias Wolf <dev-NTEO@vplace.de>
Acked-by: John Crispin <john@phrozen.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14662/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>