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>