iio: adc: at91-sama5d2_adc: add support for temperature sensor

iio: adc: at91-sama5d2_adc: add support for temperature sensor

The ADC on SAMA7G5 has a dedicated channel (channel 31) for measuring
in-SoC temperature. 2 inputs are multiplexed on channel 31, VTEMP and
VBG as follows:

          `
          | \       +-----+
VBG   --->|  | ch31 |     |
Vtemp --->|  |----->| ADC |
          |  /      |     |
          | /       +-----+
          .

where:
- VTEMP is proportional to the absolute temperature voltage
- VBG is a quasi-temperature independent voltage

Both VBG and VTEMP are needed to determine the correct in-SoC
temperature. At a moment of time only one of these could be measured, the
selection being done with bit SRCLCH bit of ACR register. The formula to
calculate the temperature is as follows:

P1 + (Vref * (VTEMP - P6 - P4 * VBG)) / (VBG * VTEMP_DT)

where:
- P1, P4, P6 are calibration data retrieved from OTP memory
- Vref is the reference voltage for ADC
- VTEMP_DT is the voltage sensitivity to temperature and is constant
- VTEMP, VBG are the measured values from channel 31

For better resolution before reading the temperature certain settings
for oversampling ratio, sample frequency, EMR.TRACKX, MR.TRACKTIM are
applied. The initial settings are reapplied at the end of temperature
reading.

Current support is not integrated with trigger buffers channel 31 not
being enabled/disabled in functions at91_adc_buffer_prepare(),
at91_adc_buffer_postdisable() thus the conversion for channel 31 is not
done in case trigger buffers are enabled. In case of trigger buffers are
enabled and temperature requests are received in the driver though
at91_adc_read_temp() the at91_adc_read_temp() will return with an error
code.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220803102855.2191070-18-claudiu.beznea@microchip.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>