riscv: sbi: Correct sbi_shutdown() and sbi_clear_ipi() export

Fix incorrect EXPORT_SYMBOL().

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>