lib: sbi: Add separate entries for firmware RX and RW regions

Add two entries for firmware in the root domain:

1. TEXT: fw_start to _fw_rw_offset with RX permissions
2. DATA: _fw_rw_offset to fw_size with RW permissions

These permissions are still not enforced from M-mode but lay
the ground work for enforcing them for M-mode. SU-mode don't
have any access to these regions.

Sample output:
 Domain0 Region01  : 0x0000000080000000-0x000000008001ffff M: (R,X) S/U: ()
 Domain0 Region02  : 0x0000000080020000-0x000000008003ffff M: (R,W) S/U: ()

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