of: remove struct property.unique_id for FDT

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 <[email protected]>
Reviewed-by: Frank Rowand <[email protected]>
Acked-by: Grant Likely <[email protected]>
Signed-off-by: Rob Herring <[email protected]>