MIPS: Add helper function to allow platforms to point at a DTB.

Add __dt_setup_arch() that can be called to load a builtin DT.
Additionally we add a macro to allow loading a specific symbol
from the __dtb_* section.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3715/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>