of: remove unused extern reference to devtree_lock

Neither the powerpc nor the microblaze code use devtree_lock anymore.
Remove the extern reference.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Michal Simek <monstr@monstr.eu>

arch/microblaze/include/asm/prom.h

@@ -30,8 +30,6 @@ extern struct device_node *of_chosen;
 
 #define HAVE_ARCH_DEVTREE_FIXUPS
 
-extern rwlock_t devtree_lock;	/* temporary while merging */
-
 /* Other Prototypes */
 extern int early_uartlite_console(void);
 

arch/powerpc/kernel/prom.c

@@ -67,8 +67,6 @@ int __initdata iommu_force_on;
 unsigned long tce_alloc_start, tce_alloc_end;
 #endif
 
-extern rwlock_t devtree_lock;	/* temporary while merging */
-
 /* export that to outside world */
 struct device_node *of_chosen;