MIPS: Lantiq: Make it possible to build in no device tree

Now it is possible to build in no device tree at all and depend on the
boot loader providing one or someone concatenating a device tree to the
end of the image.

This was copied from arch/mips/bmips/Kconfig

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: John Crispin <john@phrozen.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12899/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>