riscv: only select serial sifive if TTY is enabled

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>