of/fdt: Don't copy garbage after "/" in root node path

The root node path must be internally converted to "/", or various
pieces of code looking for it that way will fail. The code to do
that however had a bug where we might incorrectly append pieces
of the original path from the fdt to the "/".

We should probably add a proper dedicated accessor for the root node
but in the meantime this patch should fix it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>