MIPS: ingenic: Add support for appended devicetree

Add support for booting the kernel from an externally-appended
devicetree, if no devicetree was built-in.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org