spl: fit: don't load the firmware twice

spl: fit: don't load the firmware twice

When u-boot.img is a FIT image generated automatically by mkimage, the
configuration node has the following structure:
conf-1 {
   description = "k3-am654-base-board";
   firmware = "firmware-1";
   loadables = "firmware-1";
   fdt = "fdt-1";
};

The firmware is referenced twice. Once by the 'firmware' property and
once by the 'loadables' property. Currently this result in the firmware
being loaded twice. This is not a big problem but has an impact on the
boot time.
Fixing it by not loading a loadable image if it is also the firmware image.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Andreas Dannenberg <dannenberg@ti.com>