mtd: nand: pxa3xx: Add documentation about the controller

Given there's no public specification to this date, and in order
to capture some important details and singularities about the
controller let's document them once and for good.

Cc: linux-doc@vger.kernel.org
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
diff --git a/drivers/mtd/nand/pxa3xx_nand.c b/drivers/mtd/nand/pxa3xx_nand.c
index 4b3aaa8..f7c004d 100644
--- a/drivers/mtd/nand/pxa3xx_nand.c
+++ b/drivers/mtd/nand/pxa3xx_nand.c
@@ -7,6 +7,8 @@
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 2 as
  * published by the Free Software Foundation.
+ *
+ * See Documentation/mtd/nand/pxa3xx-nand.txt for more details.
  */
 
 #include <linux/kernel.h>