riscv: Cleanup sbi function stubs when RISCV_SBI disabled

Fix sbi_init() function declaration mismatch between RISCV_SBI
enable and disable, as it always returned 0, make it void function.

Drop some stubs which won't be used if RISCV_SBI disabled.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>