Battery temperature ADC plumbing on Qualcomm platforms

This is an RFC which implements a potential solution to get battery
temperature readings working on for example smartphones with Qualcomm
SoCs.

The solution chosen in downstream Qualcomm kernels is exposing
ADC_BAT_THERM_PU* in the ADC driver as temperature channels with the
lookup table ("struct vadc_map_pt") for the specific NTC found in a
device's battery patched to adjust the lookup table.

The high level solution proposed here:
* ADC driver provides temperature channel in (milli)volt as IIO channel
* generic-adc-thermal driver converts voltage to temperature based on
  provided lookup table from DT (driver has one IIO channel input, one
  IIO channel output)
* The fuel gauge driver can use that temperature IIO channel to expose
  battery temperature via the power supply device

Variants/alternatives considered:

1. Do not implement IIO device in generic-adc-thermal

Without an IIO channel and using thermal zone directly it becomes more
difficult. You cannot get thermal zone by reference (e.g.
thermal-sensors = <&foo>;). The function thermal_zone_get_zone_by_name()
exists but lookup by name is kinda clunky. Adding by-phandle support
might be possible but is lots of work. It also doesn't really look like
thermal-sensor is really meant to be used by other drivers. E.g.
there's also no "thermal-sensor-names" property to designate passing
multiple thermal sensors. So I believe IIO is a better fitting API for
this.

2. Expose IIO channel as temperature in ADC driver

This would require passing in the temperature lookup table somehow to
the ADC driver via DT. I think this passes too many details about the
hardware that's connected into the ADC driver. While possible, at least
for Qcom ADC there's no precedent yet.

3. Add temperature-lookup-table as property to simple-battery

Since the NTC is a part of the battery pack, adding a
temperature-lookup-table property to simple-battery would make sense
instead of having this lookup table be standalone in the
generic-adc-thermal node. However being able to re-use the existing code
in generic-adc-thermal lead me to the current proposal.

Signed-off-by: Luca Weiss <luca.weiss@fairphone.com>
---
Luca Weiss (6):
      iio: adc: qcom-spmi-adc5: Add battery thermal channels
      dt-bindings: thermal: generic-adc: Document #io-channel-cells
      thermal/drivers/generic-adc: Register as IIO device
      thermal/drivers/generic-adc: Allow probe without TZ registration
      arm64: dts: qcom: pm7250b: Define battery temperature ADC channels
      arm64: dts: qcom: sm7225-fairphone-fp4: Add battery temperature node

 .../bindings/thermal/generic-adc-thermal.yaml      |  4 ++
 arch/arm64/boot/dts/qcom/pm7250b.dtsi              | 24 +++++++
 arch/arm64/boot/dts/qcom/sm7225-fairphone-fp4.dts  | 83 ++++++++++++++++++++++
 drivers/iio/adc/qcom-spmi-adc5.c                   |  6 ++
 drivers/iio/adc/qcom-vadc-common.c                 | 16 +++++
 drivers/thermal/thermal-generic-adc.c              | 68 ++++++++++++++++--
 include/linux/iio/adc/qcom-vadc-common.h           |  3 +
 7 files changed, 198 insertions(+), 6 deletions(-)
---
base-commit: 6063257da111c7639d020c5f15bfb37fb839d8b6
change-id: 20251010-bat-temp-adc-8539bf0b85bc

Best regards,
-- 
Luca Weiss <luca.weiss@fairphone.com>