libpayload: Export device count in storage interface

FILO can use this as offset to enumerate AHCI and its own IDE
devices together.

Change-Id: I57380e7bd1df6db5c882427e9a34d068f4348fb2
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: http://review.coreboot.org/1846
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

payloads/libpayload/include/storage/storage.h

@@ -68,6 +68,7 @@ typedef struct storage_dev {
 	void (*detach_device)(struct storage_dev *);
 } storage_dev_t;
 
+int storage_device_count(void);
 int storage_attach_device(storage_dev_t *dev);