riscv: sbi: Fix undefined reference to sbi_shutdown

There is no shutdown call in SBI v0.2, only set pm_power_off
when RISCV_SBI_V01 enabled to fix following build error,

riscv64-linux-ld: arch/riscv/kernel/sbi.o: in function `sbi_power_off':
sbi.c:(.text+0xe): undefined reference to `sbi_shutdown

Fixes: efca1398 ("RISC-V: Introduce a new config for SBI v0.1")
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>