SPL: FIT: factor out spl_load_fit_image()

SPL: FIT: factor out spl_load_fit_image()

At the moment we load two images from a FIT image: the actual U-Boot
image and the .dtb file. Both times we have very similar code, that deals
with alignment requirements the media we load from imposes upon us.
Factor out this code into a new function, which we just call twice.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Jagan Teki <jagan@openedev.com>