lib: Add system early_exit and final_exit APIs

This patch adds system-level early_exit and final_exit APIs
with corresponding platform hooks. These new APIs will be
primarily used by sbi_exit() in OpenSBI exit path.

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