of/fdt: convert initial_boot_params to opaque pointer

Now that all accesses to FDT header data has been converted to accessor
helpers, initial_boot_params can become an opaque pointer.

Signed-off-by: Rob Herring <robh@kernel.org>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Tested-by: Grant Likely <grant.likely@linaro.org>
Tested-by: Stephen Chivers <schivers@csc.com>