qemu: Support optional encryption of BL31 and BL32 images

Enable encryption IO layer to be stacked above FIP IO layer for optional
encryption of Bl31 and BL32 images in case ENCRYPT_BL31 or ENCRYPT_BL32
build flag is set.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Change-Id: I24cba64728861e833abffc3d5d9807599c49feb6
3 files changed