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>