DM: crypto/rsa_mod_exp: Add rsa Modular Exponentiation DM driver

Add a new rsa uclass for performing modular exponentiation and implement
the software driver basing on this uclass.

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/drivers/crypto/rsa_mod_exp/Kconfig b/drivers/crypto/rsa_mod_exp/Kconfig
new file mode 100644
index 0000000..6dcb39a
--- /dev/null
+++ b/drivers/crypto/rsa_mod_exp/Kconfig
@@ -0,0 +1,5 @@
+config DM_MOD_EXP
+	bool "Enable Driver Model for RSA Modular Exponentiation"
+	depends on DM
+	help
+	  If you want to use driver model for RSA Modular Exponentiation, say Y.