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: NNicolas Pitre <nico@linaro.org>
Reviewed-by: NFrank Rowand <frowand.list@gmail.com>
Acked-by: NGrant Likely <grant.likely@secretlab.ca>
Signed-off-by: NRob Herring <robh@kernel.org>

include/linux/of.h

@@ -38,7 +38,9 @@ struct property {
 	void	*value;
 	struct property *next;
 	unsigned long _flags;
+#if defined(CONFIG_OF_PROMTREE)
 	unsigned int unique_id;
+#endif
 	struct bin_attribute attr;
 };