andes: config: add riscv falcon mode for ae350 platform

Fork from ae350_rv[32/64]_spl_[xip]_defconfig and
append CONFIG_SPL_LOAD_FIT_OPENSBI_OS_BOOT=y

Signed-off-by: Randolph <randolph@andestech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>