sunxi: binman: Update FIT component descriptions

Since commit d879616e ("spl: fit: simplify logic for FDT loading for
non-OS boots"), the SPL looks at the "os" properties of FIT images to
determine where to append the FDT.

The "os" property of the "firmware" image also determines how to execute
the next stage of the boot process, as in 1d379090 ("spl: atf:
introduce spl_invoke_atf and make bl31_entry private"). For this reason,
the next stage must be specified in "firmware", not in "loadables".

To support this additional functionality, and to properly model the boot
process, where ATF runs before U-Boot, add the "os" properties and swap
the firmware/loadable images in the FIT image.

Since this description was copied as an example in commit 70248d6a2916
("binman: Support generating FITs with multiple dtbs"), update those
examples as well for correctness and consistency.

Acked-by: Patrick Wildt <patrick@blueri.se>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>