lib: sbi: Add Smstateen extension defines

Smstateen extension provides a mechanism to plug potential
covert channels which are opened by extensions that add to
processor state that may not get context-switched.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>