firmware: Add relocatable FW_PAYLOAD_FDT_ADDR

The fw_payload.bin has the same issue as described in previous patch.
But only FW_PAYLOAD_FDT_ADDR is affected.

Add FW_PAYLOAD_FDT_OFFSET to identify relocatable payload fdt address.

Signed-off-by: Inochi Amaoto <inochiama@outlook.com>
Reviewed-by: Anup Patel <anup@brainfault.org>