MIPS: lantiq: unbreak devicetree init

The bootmem was incorrectly freed resulting in lots of dangling pointers.
Additionally we should use of_platform_populate() as the Documentaion tells us
to do so.

Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4518