lib: Introduce sbi_ipi_send_smode() API

Instead of directly calling sbi_ipi_send_many(), we introduce
sbi_ipi_send_smode() for injecting S-mode software interrupts.

This way in future we can assign any IPI event number for S-mode
IPIs within sbi_ipi.c only.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>