include: sbi: Fine grain the permissions for M and SU modes

Split the permissions for M-mode and SU-mode. This would
help if different sections of OpenSBI need to be given
different permissions and if M-mode has different permisssions
than the SU-mode over a region.

Signed-off-by: Himanshu Chauhan <hchauhan@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Tested-by: Anup Patel <anup@brainfault.org>