dt-bindings: mtd: fixed-partitions: Add compression property

Sometimes the contents of a partition are compressed. Add a property to
express this and define the algorithm used.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230927180545.3522628-1-sjg@chromium.org