riscv: enter WFI in default_power_off() if SBI does not shutdown

Provide a new default fallback power off that just sits in a wfi loop
to save some power.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
[paul.walmsley@sifive.com: split the WFI fix apart from the
 nommu-related default_power_off() changes]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

arch/riscv/kernel/reset.c

@@ -10,7 +10,8 @@
 static void default_power_off(void)
 {
 	sbi_shutdown();
-	while (1);
+	while (1)
+		wait_for_interrupt();
 }
 
 void (*pm_power_off)(void) = default_power_off;