dt-bindings: mtd: partitions: Change qcom,smem-part partition type

As described in dd638202 ("dt-bindings: mtd: partitions: add additional
example for qcom,smem-part"), the aim of documenting the subnodes was to be
able to declare nvmem cells. Hence, the partition property does not
really apply directly here, let's instead reference nvmem-cells.yaml
first.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20221114090315.848208-13-miquel.raynal@bootlin.com