of/device: Make of_device_make_bus_id() usable by other code.

The AMBA bus should also use of_device_make_bus_id() when populating device
out of device tree data.  This patch makes the function non-static, and
adds a suitable prototype in of_device.h

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
diff --git a/drivers/of/platform.c b/drivers/of/platform.c
index 033a224..30a4641 100644
--- a/drivers/of/platform.c
+++ b/drivers/of/platform.c
@@ -508,7 +508,7 @@
  * value to derive a unique name.  As a last resort it will use the node
  * name followed by a unique number.
  */
-static void of_device_make_bus_id(struct device *dev)
+void of_device_make_bus_id(struct device *dev)
 {
 	static atomic_t bus_no_reg_magic;
 	struct device_node *node = dev->of_node;