fit: Support compression for non-kernel components (e.g. FDT)

This patch adds support for compressing non-kernel image nodes in a FIT
image (kernel nodes could already be compressed previously). This can
reduce the size of FIT images and therefore improve boot times
(especially when an image bundles many different kernel FDTs). The
images will automatically be decompressed on load.

This patch does not support extracting compatible strings from
compressed FDTs, so it's not very helpful in conjunction with
CONFIG_FIT_BEST_MATCH yet, but it can already be used in environments
that select the configuration to load explicitly.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>