of/fdt: Clean up casting in unflattening path

The flat tree unflatting path is using unsigned longs to carry around
virtual address pointers to the device tree and the allocated memory
used to unpack it. This is a little insane since every access to them
needs to be cast to a pointer type before using it. This patch changes
the data type to void* for the 'start' and 'mem' pointers and reworks
the unflattening functions to use those values directly which results in
slightly simpler code.

Signed-off-by: Grant Likely <grant.likely@linaro.org>