MIPS: BMIPS: Support APPENDED_DTB

Use appended DTB when available.

Signed-off-by: Jaedon Shin <jaedon.shin@gmail.com>
Cc: Kevin Cernekee <cernekee@gmail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14337/


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