firmware: Split RO/RX and RW sections

Split the RO/RX and RW sections so that they can have
independent pmp entries with required permissions. The
split size is ensured to be a power-of-2 as required by
pmp.

_fw_rw_offset symbol marks the beginning of the data
section.

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