SPL: SPI: sunxi: add SPL FIT image support

The sunxi-specific SPI load routine only knows how to load a legacy
U-Boot image.
Teach it how to handle FIT images as well, simply by providing the
existing SPL FIT loader with the right loader routine to access the SPI
NOR flash.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reported-by: Peter Kosa <kope@madnet.sk>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>