iio: adc: stm32: add oversampling support

Add oversampling support for STM32H7, STM32MP15 & STM32MP13.
STM32F4 ADC has no oversampling feature.

The current support of the oversampling feature aims at increasing the
data SNR, without changing the data resolution.
As the oversampling by itself increases data resolution, a right shift
is applied to keep the initial resolution.
Only the oversampling ratio corresponding to a power of two are
supported here, to get a direct link between right shift and
oversampling ratio. (2^n ratio <=> n right shift)

The oversampling ratio is shared by all channels, whatever channel type.
(e.g. single ended or differential).

Oversampling can be configured using IIO ABI:
- oversampling_ratio_available
- oversampling_ratio

Co-developed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://patch.msgid.link/20250424151604.626758-1-olivier.moysan@foss.st.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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