iio: adc: meson-saradc: add support for the chip's temperature sensor

Channel 6 of the SAR ADC can be switched between two inputs:
SAR_ADC_CH6 input (an actual pad on the SoC) and the signal from the
temperature sensor inside the SoC.

To get usable results from the temperature sensor we need to read the
corresponding calibration data from the eFuse and pass it to the SAR ADC
registers. If the temperature sensor is not calibrated (the eFuse data
contains a bit for this) then the driver will only register the
iio_chan_spec's for voltage measurements.

This only enables the temperature sensor for the Meson8 SoC. Meson8b and
Meson8m2 SoCs can be supported in the future as well but we first need
a way to pass the fifth TSC (temperature sensor coefficient) bit to the
HHI register area (apart from that the infrastructure as already
implemented for Meson8 can be used). On the 64-bit SoCs (GXBB, GXL and
GXM) the temperature sensor inside SAR ADC is firmware-controlled (by
BL30, we can simply use the SCPI hwmon driver to get the chip
temperature).

To keep the devicetree interface backwards compatible we simply skip the
temperature sensor initialization if no eFuse nvmem cell is passed via
devicetree.

The public documentation for the SAR ADC IP block does not explain how
to use the registers to read the temperature. The logic from this patch
is based on reading and understanding Amlogic's GPL kernel sources.
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>