[MIPS] Ranchu platform Appended DTB support

[MIPS] Ranchu platform Appended DTB support

With CONFIG_MIPS_APPENDED_DTB=y option enabled, the boot code
will look for a device tree binary (dtb) appended to raw
vmlinux.bin (without decompressor).
(e.g. cat vmlinux.bin <filename>.dtb > vmlinux_w_dtb).

Beware that there is very little in terms of protection against
this option being confused by leftover garbage in memory that might
look like a DTB header after a reboot if no actual DTB is appended
to vmlinux.bin.

Change-Id: Ia1d7e0e4fe3848385b84cb06b04ea480897946f9
Reviewed-on: https://mipsia.review.mips.com/2860
Reviewed-by: Raghu Gandham <raghu.gandham@imgtec.com>
Tested-by: Raghu Gandham <raghu.gandham@imgtec.com>