riscv: cleanup the default power off implementation

Move the sbi poweroff to a separate function and file that is only
compiled if CONFIG_SBI is set.

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 into a separate patch]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>