fdtdec: Support retrieving the name of a carveout

When retrieving a given carveout for a device, allow callers to query
the name. This helps differentiating between carveouts when there are
more than one.

This is also useful when copying carveouts to help assign a meaningful
name that cannot always be guessed.

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