of: remove struct property.unique_id for FDT

Only Sparc uses unique_id, so remove it for FDT builds and shrink struct
property a bit making the unflattened DT less of a memory hog.

Tested-by: Nicolas Pitre <nico@linaro.org>
Reviewed-by: Frank Rowand <frowand.list@gmail.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Rob Herring <robh@kernel.org>