riscv: Avoid calling sbi_clear_ipi()

riscv: Avoid calling sbi_clear_ipi()

There is no need for S-mode U-Boot to call sbi_clear_ipi() as it
can be cleared directly from S-mode. This saves some cycles.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas@auer.io>