lib: sbi: Grant SU R/W/X permissions to whole memory

Since pmp entries have implicit priority on index, previous entries will
deny access to SU on M-mode region. Also, M-mode will not have access to
SU region while previous entries will allow access to M-mode regions.

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