lib: sbi: Use finer permission sematics to decide on PMP bits

Use the fine grained permission bits to decide if the region
permissions are to be enforced on all modes. Also use the new
permission bits for deciding on R/W/X bits in pmpcfg register.

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