dt-bindings: mtd: fixed-partitions: Move "compression" to partition node

dt-bindings: mtd: fixed-partitions: Move "compression" to partition node

The "compression" property is defined in the wrong place as it applies
to individual partitions nodes, not all nodes.

Fixes: 8baba8d52ff5 ("dt-bindings: mtd: fixed-partitions: Add compression property")
Reviewed-by: Simon Glass <simon.glass@canonical.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>