SPL: Add FIT data-position property support

For external data, FIT has a optional property "data-position" which
can set the external data to a fixed offset to FIT beginning.
Add the support for this property in SPL FIT.

Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tomas Melin <tomas.melin@vaisala.com>
Cc: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Cc: "Andrew F. Davis" <afd@ti.com>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: "tomas.melin@vaisala.com" <tomas.melin@vaisala.com>
Cc: Kever Yang <kever.yang@rock-chips.com>
Cc: Andre Przywara <andre.przywara@arm.com>
Cc: York Sun <york.sun@nxp.com>
Cc: Lokesh Vutla <lokeshvutla@ti.com>
Cc: "Cooper Jr., Franklin" <fcooper@ti.com>
Cc: George McCollister <george.mccollister@gmail.com>
Cc: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Cc: Jean-Jacques Hiblot <jjhiblot@ti.com>
Cc: Rick Altherr <raltherr@google.com>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: York Sun <york.sun@nxp.com>