lib: Introduce sbi_ipi_event_create/destroy() APIs

This patch introduces sbi_ipi_event_create/destroy() APIs and
struct sbi_ipi_event_ops for creating/destroying IPI events
at runtime based of event operations.

This new APIs will help platform code and utils code to create
custom IPI events which are not part of generic OpenSBI library.

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