dt-bindings: mtd: Ensure partition node properties are documented

dt-bindings: mtd: Ensure partition node properties are documented

Partition nodes without a compatible have no restrictions on additional
properties. Fix this by ensuring 'unevaluatedProperties' is set when
there's no compatible property. If there is a compatible property, then
there should be a specific schema for it which will enforce
'unevaluatedProperties'.

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>