spl: Support loading a FIT from FAT FS

Detect a FIT when loading from a FAT File system and handle it using the
new FIT SPL support.

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>