fit: Do not automatically decompress ramdisk images

The Linux ramdisk should always be decompressed by the kernel itself,
not by U-Boot. Therefore, the 'compression' node in the FIT image should
always be set to "none" for ramdisk images, since the only point of
using that node is if you want U-Boot to do the decompression itself.

Yet some systems populate the node to the compression algorithm used by
the kernel instead. This used to be ignored, but now that we support
decompression of all image types it becomes a problem. Since ramdisks
should never be decompressed by U-Boot anyway, this patch adds a special
exception for them to avoid these issues. Still, setting the
'compression' node like that is wrong in the first place, so we still
want to print out a warning so that third-party distributions doing this
can notice and fix it.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
Tested-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>