riscv: Add S-mode defconfigs for QEMU virt machine

This patch adds S-mode defconfigs for QEMU virt machine so
that we can run u-boot in S-mode on QEMU using M-mode runtime
firmware (BBL or equivalent).

Signed-off-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>