uboot-phytec

doc: FIT image: clarify usage of "compression" property

Compressed images should have their compression property
set to "none" if U-Boot should leave them compressed.

This is especially the case for compressed ramdisks that
should be uncompressed by the kernel only.

Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>