lib: Add initial sbi_exit() API

This patch adds initial implementation of sbi_exit() API which
can be used to perform OpenSBI exit sequence for current HART.

The sbi_exit() implementation will be further extended by
subsequent patches.

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