of/fdt: Make fdt blob input parameters of unflatten functions const

Operations to unflatten fdt blobs never modify the input blobs, hence
make them const. Now we no longer need to cast arbitrary const data to
"void *" when calling of_fdt_unflatten_tree().

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>