lib: Introduce sbi_ipi_send_halt() API

Instead of directly calling sbi_ipi_send_many(), we introduce
sbi_ipi_send_halt() for halting a set of HARTs.

This way in future we can assign any IPI event number for HART
halting within sbi_ipi.c only.

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