(kernel-rolling) iio: adc: Add Phytium ADC support

[MingcongBai]

Picked and rebased from #193.

From original pull request:

Phytium Pe220x SoCs includes an 8-channel, 10-bit single ended ADC. This patch add this ADC's driver support.

Builds tested

• amd64
• arm64
• loong64

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Phytium ADC driver

This driver is for Phytium's ADC module. It provides a simple IIO driver for
reading and writing ADC data. The driver supports multi-channel ADC
conversions and provides a threshold detection feature.

To build the driver, the following steps are needed:

1. Copy the code to your kernel tree.
2. Modify the driver structure to include the new ADC driver.
3. Update the driver name in the platform driver structure.
4. Implement the necessary functions for reading, writing, and configuring
   the ADC module.
5. Add any necessary platform-specific code, such as GPIO configuration or
   clock initialization.
6. Implement the necessary interrupt handling, if required.
7. Add any necessary debugging code.

This driver is designed to be used with the Phytium ADC module. It provides a
simple IIO driver for reading and writing ADC data. The driver supports
multi-channel ADC conversions and provides a threshold detection feature.