mips: convert fdt pointers to opaque pointers

The architecture code does not need to access the internals of the FDT
blob directly, so make the pointers to it void * and use char arrays
for section variables.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Tested-by: Grant Likely <grant.likely@linaro.org>