fdt_support: fdt_translate_address() blob const correctness

The next patch will call fdt_translate_address() from somewhere with a
"const void *blob" rather than a "void *blob", so fdt_translate_address()
must accept a const pointer too. Constify the minimum number of function
parameters to achieve this.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Squashed in build fix from Stephen:
Signed-off-by: Simon Glass <sjg@chromium.org>