dt-bindings: iio: stm32-adc: add nvmem support for vrefint internal channel

Add support of nvmem. This allows to retrieve calibration data from OTP
for vrefint internal channel.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211014131228.4692-3-olivier.moysan@foss.st.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>