spl: Allow to load a FIT containing U-Boot from FS

spl: Allow to load a FIT containing U-Boot from FS

This provides a way to load a FIT containing U-Boot and a selection of device
tree files from a File system. Making sure that all the reads and writes
are aligned to their respective needs.

Tested-by: Michal Simek <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Reviewed-by: Tom Rini <[email protected]>
Signed-off-by: Lokesh Vutla <[email protected]>
[trini: Make this still apply with Michal's alignment change for 'fit']
Signed-off-by: Tom Rini <[email protected]>