iio: adc: Add Allwinner D1/T113s/R329/T507 SoCs GPADC

The General Purpose ADC (GPADC) can convert the external signal into
a certain proportion of digital value, to realize the measurement of
analog signal, which can be applied to power detection and key detection.

Theoretically, this ADC can support up to 16 channels. All SoCs below
contain this GPADC IP. The only difference between them is the number
of available channels:

 T113 - 1 channel
 D1   - 2 channels
 R329 - 4 channels
 T507 - 4 channels

Reviewed-by: Andy Shevchenko <[email protected]>
Signed-off-by: Maksim Kiselev <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Cameron <[email protected]>

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