c6x: convert fdt pointers to opaque pointers

The architecture code does not need to access the internals of the FDT
blob, so make the pointer to it void *.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Mark Salter <msalter@redhat.com>
Cc: Aurelien Jacquiot <a-jacquiot@ti.com>
Cc: linux-c6x-dev@linux-c6x.org
Tested-by: Grant Likely <grant.likely@linaro.org>