include: sbi: macros for mseccfg.sseed and .useed

Define macros to access the sseed and the useed bit in the machine
security configuration register (mseccfg).

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>