riscv: Kconfig: Add SPL_ZERO_MEM_BEFORE_USE

Add a Kconfig item to allow SPL to clear stack/GD/malloc area before
using them.

Signed-off-by: Bo Gan <ganboing@gmail.com>
Signed-off-by: Shengyu Qu <wiagn233@outlook.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>