MIPS: Lantiq: Add support for device tree file from boot loader

This fetches the device tree file like it is specified in the MIPS UHI
interface if one was found. This is also used when the device tree file
was appended to the kernel image with cat.
This code is copied from arch/mips/bmips/setup.c.

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/12898/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>