riscv: Enable idle generic idle loop

Enable generic idle loop to support for hlt/nohlt command line options
to override default idle loop behavior.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/Kconfig

@@ -48,6 +48,7 @@ config RISCV
 	select GENERIC_CLOCKEVENTS_BROADCAST if SMP
 	select GENERIC_EARLY_IOREMAP
 	select GENERIC_GETTIMEOFDAY if HAVE_GENERIC_VDSO
+	select GENERIC_IDLE_POLL_SETUP
 	select GENERIC_IOREMAP
 	select GENERIC_IRQ_MULTI_HANDLER
 	select GENERIC_IRQ_SHOW