On the older-gen 32-bit SoCs the meson-saradc driver is used to read the
SoC temperature. This requires reading calibration data from the eFuse.

Looking up the calibration data nvmem-cell requires the OF device_node
pointer to be available in the struct device which is passed to
devm_nvmem_cell_get(). This however got lost with commit 8cb631cc
("iio: Remove superfluous of_node assignments") from indio_dev->dev. As
devm_nvmem_cell_get() is called in the initialization phase the
device_node is not yet available because the NVMEM cell is looked up
before iio_device_register() is called (which would then set the
device_node automatically).
Use the parent device to look up the NVMEM cell instead to fix this
issue.

Fixes: 8cb631cc ("iio: Remove superfluous of_node assignments")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>