fdtdec: Allow using fdtdec_get_carveout() in loops

In order make it possible to use fdtdec_get_carveout() in loops, return
FDT_ERR_NOTFOUND when the passed-in index exceeds the number of phandles
present in the given property.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>