firmware: Add RW section offset in scratch

Add the RW section offset, provided by _fw_rw_offset symbol,
to the scratch structure. This will be used to program
separate pmp entry for RW section.

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