Blackfin arch: Add document about bfin-gpio

Add document about bfin-gpio when requesting a pin
both as gpio and gpio interrupt.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>

diff --git a/Documentation/blackfin/00-INDEX b/Documentation/blackfin/00-INDEX
index 7cb3b35..d6840a9 100644
--- a/Documentation/blackfin/00-INDEX
+++ b/Documentation/blackfin/00-INDEX
@@ -9,3 +9,6 @@
 
 Filesystems
 	- Requirements for mounting the root file system.
+
+bfin-gpio-note.txt
+	- Notes in developing/using bfin-gpio driver.