sparc: Move 'unique_id' into prom_common.c and rename to 'prom_unique_id'

This will be used in a subsequent changeset.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/arch/sparc/kernel/prom.h b/arch/sparc/kernel/prom.h
index 4992510..2575cdb 100644
--- a/arch/sparc/kernel/prom.h
+++ b/arch/sparc/kernel/prom.h
@@ -13,4 +13,6 @@
 extern void irq_trans_init(struct device_node *dp);
 #endif
 
+extern unsigned int prom_unique_id;
+
 #endif /* __PROM_H */