There is some warning if TTY is not enabled, and lead to
build error, only select serial sifive if TTY enabled, and
this also makes randconfig happy.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
[paul.walmsley@sifive.com: updated to apply]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>