rsa : Compile Modular Exponentiation files based on CONFIG_RSA_SOFTWARE_EXP

Remove dependency of rsa_mod_exp from CONFIG_FIT_SIGNATURE.
As rsa modular exponentiation is an independent module
and can be invoked independently.

Signed-off-by: Gaurav Rana <gaurav.rana@freescale.com>
Acked-by: Simon Glass <sjg@chromium.org>
Reviewed-by: York Sun <yorksun@freescale.com>
diff --git a/lib/rsa/Makefile b/lib/rsa/Makefile
index cc25b3c..6867e50 100644
--- a/lib/rsa/Makefile
+++ b/lib/rsa/Makefile
@@ -7,4 +7,5 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-$(CONFIG_FIT_SIGNATURE) += rsa-verify.o rsa-checksum.o rsa-mod-exp.o
+obj-$(CONFIG_FIT_SIGNATURE) += rsa-verify.o rsa-checksum.o
+obj-$(CONFIG_RSA_SOFTWARE_EXP) += rsa-mod-exp.o