arm: fdt: Relocate fdt along with other data

Rather than leave the fdt down next to the code/data, we really should
relocate it along with everything else. For CONFIG_OF_EMBED this happens
automatically, but for CONFIG_OF_SEPARATE it does not.

Add code to copy the fdt and point to the new copy after relocation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>