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>

Author: Kefeng Wang

arch/riscv/Kconfig.socs

@@ -2,8 +2,8 @@ menu "SoC selection"
 
 config SOC_SIFIVE
 	bool "SiFive SoCs"
-	select SERIAL_SIFIVE
-	select SERIAL_SIFIVE_CONSOLE
+	select SERIAL_SIFIVE if TTY
+	select SERIAL_SIFIVE_CONSOLE if TTY
 	select CLK_SIFIVE
 	select CLK_SIFIVE_FU540_PRCI
 	select SIFIVE_PLIC