lib/rsa: Modify rsa to use DM driver

Modify rsa_verify to use the rsa driver of DM library .The tools
will continue to use the same RSA sw library.

CONFIG_RSA is now dependent on CONFIG_DM. All configurations which
enable FIT based signatures have been modified to enable CONFIG_DM
by default.

Signed-off-by: Ruchika Gupta <ruchika.gupta@freescale.com>
CC: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/README b/README
index fefa71c..cac7978 100644
--- a/README
+++ b/README
@@ -3176,8 +3176,13 @@
 		This enables the RSA algorithm used for FIT image verification
 		in U-Boot. See doc/uImage.FIT/signature.txt for more information.
 
+		The Modular Exponentiation algorithm in RSA is implemented using
+		driver model. So CONFIG_DM needs to be enabled by default for this
+		library to function.
+
 		The signing part is build into mkimage regardless of this
-		option.
+		option. The software based modular exponentiation is built into
+		mkimage irrespective of this option.
 
 - bootcount support:
 		CONFIG_BOOTCOUNT_LIMIT