spl: Support loading a FIT from NAND

Detect a FIT when loading from NAND and handle it using the
new FIT SPL support.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
[trini: Make sure we continue to use (void *)(unsigned long) for
load_addr].
Signed-off-by: Tom Rini <trini@konsulko.com>