lib/rsa: Add Kconfig for devices supporting RSA Modular Exponentiation

Kconfig option added for devices which support RSA Verification.
1. RSA_SOFTWARE_EXP
Enables driver for supporting RSA Modular Exponentiation in Software
2. RSA_FREESCALE_EXP
Enables driver for supporting RSA Modular Exponentiation using Freescale specific
driver

The above drivers use RSA uclass

Signed-off-by: Ruchika Gupta <ruchika.gupta@freescale.com>
CC: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
(Removed duplicate line in Kconfig comment)

Change-Id: I7663c4d5350e2bfc3dfa2696f70ef777d6ccc6f6