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 <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
[trini: Make this still apply with Michal's alignment change for 'fit']
Signed-off-by: Tom Rini <trini@konsulko.com>