SPL: Add FIT data-position property support

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 <[email protected]>
Signed-off-by: Peng Fan <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Reviewed-by: Tomas Melin <[email protected]>
Cc: Pantelis Antoniou <[email protected]>
Cc: "Andrew F. Davis" <[email protected]>
Cc: Igor Grinberg <[email protected]>
Cc: "[email protected]" <[email protected]>
Cc: Kever Yang <[email protected]>
Cc: Andre Przywara <[email protected]>
Cc: York Sun <[email protected]>
Cc: Lokesh Vutla <[email protected]>
Cc: "Cooper Jr., Franklin" <[email protected]>
Cc: George McCollister <[email protected]>
Cc: Tuomas Tynkkynen <[email protected]>
Cc: Jean-Jacques Hiblot <[email protected]>
Cc: Rick Altherr <[email protected]>
Cc: Tom Rini <[email protected]>
Reviewed-by: Lokesh Vutla <[email protected]>
Reviewed-by: York Sun <[email protected]>