of: move definition of of_chosen into common code.

Rather than defining of_chosen in each arch, it can be defined for all
in driver/of/base.c

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

drivers/of/base.c

@@ -23,6 +23,7 @@
 #include <linux/proc_fs.h>
 
 struct device_node *allnodes;
+struct device_node *of_chosen;
 
 /* use when traversing tree through the allnext, child, sibling,
  * or parent members of struct device_node.