firmware: Return real DTB address when FW_xyz_FDT_ADDR is not defined

Function fw_next_arg1 in firmware/fw_jump.S:59 and
firmware/fw_payload.S:63 should return real dtb
address(if specified in a1) in a0, in case we don't
want to specify FW_xyz_FDT_ADDR when compiling.

Signed-off-by: Liu Yibin <yibin_liu@c-sky.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>