iio: adc: Add support for AVIA HX710B ADC

The HX710B is a 24-bit ADC from AVIA Semiconductor, related to the
HX711 already supported in this driver.  Unlike the HX711 (which has
selectable gain and two input channels), the HX710B has a fixed gain
of 128 and two operating modes selected by the trailing PD_SCK pulse
count after each conversion:

  25 pulses (1 trailing): differential input at 10 SPS
  26 pulses (2 trailing): DVDD-AVDD supply monitor at 40 SPS
  27 pulses (3 trailing): differential input at 40 SPS

This series first extends the existing HX711 binding and driver with
preparatory refactoring, then adds HX710B support on top.

The differential input is exposed as a single IIO channel with
IIO_CHAN_INFO_SAMP_FREQ, allowing userspace to select 10 or 40 SPS
without needing two separate channels for the same physical input.
The supply monitor is a second channel at fixed 40 SPS.

This revision is rebased onto current mainline.  Note that
iio_device_claim_direct_mode() / iio_device_release_direct_mode() were
removed from the IIO core since v10, so 11/11 now uses the replacement
iio_device_claim_direct() / iio_device_release_direct() API.

Tested on a PocketBeagle2 (TI AM625): the avia,hx710b node probes and
the driver registers its two IIO channels.

Changes from v10:
  - 02/11: Retitled to "add VSUP and DVDD supply properties"; the
    commit message now documents both supplies (the patch already
    added both; only the title/text were out of sync). [Jonathan]
  - 04/11: Reworded commit message so DVDD is no longer described as
    HX710B-specific (it is added for both parts in 02/11). The
    compatible match now uses "contains: const: avia,hx710b" so the
    constraints still apply if the compatible is later used as a
    fallback. dvdd-supply is now required for HX710B (in the HX710B
    branch only, so existing HX711 device trees are unaffected) and
    added to the HX710B example. [Jonathan, Conor, Sashiko]
  - 11/11: Kconfig title and file header use "similar" instead of
    "compatible". Simplified the tail of hx711_set_hx710b_channel()
    to "return hx711_wait_for_ready()". Dropped an unrelated
    reindentation of the iio_push_to_buffers_with_timestamp() call.
    Converted hx710b_write_raw() to the new
    iio_device_claim_direct() API (see above). [Jonathan]

The Reviewed-by (Andy Shevchenko) and Acked-by (Conor Dooley) tags on
02/11 and 04/11 are carried over although those patches changed; please
re-confirm if you disagree with the reworked constraints.

Patches 01/11, 03/11, and 05/11 are unchanged from v10.

Piyush Patle (11):
  dt-bindings: iio: adc: hx711: clean up existing binding text
  dt-bindings: iio: adc: hx711: add VSUP and DVDD supply properties
  dt-bindings: iio: adc: hx711: add RATE GPIO property
  dt-bindings: iio: adc: hx711: add HX710B support
  iio: adc: hx711: move scale computation to per-device storage
  iio: adc: hx711: introduce hx711_chip_info structure
  iio: adc: hx711: pass trailing pulse count into hx711_read
  iio: adc: hx711: split variable assignments in hx711_read and
    hx711_reset
  iio: adc: hx711: localize loop iterators in hx711_read
  iio: adc: hx711: pass iio_chan_spec to hx711_reset_read
  iio: adc: hx711: add support for HX710B

 .../bindings/iio/adc/avia-hx711.yaml          |  82 ++++-
 drivers/iio/adc/Kconfig                       |   8 +-
 drivers/iio/adc/hx711.c                       | 343 +++++++++++++++---
 3 files changed, 359 insertions(+), 74 deletions(-)

-- 
2.43.0