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/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 0111f25..660063e 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -3,4 +3,5 @@
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_FIT_SIGNATURE=y
+CONFIG_DM=y
 CONFIG_DEFAULT_DEVICE_TREE="sandbox"