Maxim MAX1241 driver

Hi everyone,

Here's version 6 of a patch series which adds support for the Maxim
MAX1241, a 12-bit, single-channel, SPI-connected ADC.

Changelog so far:

v6:

* Add separate cleanup actions for vdd and vref regulators respectively
* Change shdn pin name to shutdown. shdn is what the datasheet calls it
  but "shutdown" seems to be semi-standard, at least in the bindings
  (Rob orriginally suggested that I change the name in the docs. Figured
  it made zero sense to have it bear another name in the code)
* Dropped maxItems property in vref binding, made binding dual-licensed
* Changed name of spi node in binding example 

v5:

* Add vdd supply binding
* Reorder include headers -- one of them had escaped the long mechanized
  hand of the Alphabetron
* Drop scan_index and scan_type from channel spec, as the driver
  doesn't support buffered modes
* Drop a useless initialization of a local variable in probe function
* Revise dt-bindings patch subject line, revise maintainer and
  copyright to match the Signed-off: string, drop reference to driver
  from bindings description
* Various readability/cosmetic fixes

v4:

* Dropped explicit documentation of SPI reg property
* Reordered patch series so that dt bindings come first

v3:

* Fixed silly copy-paste error in Kconfig description

v2:

* Removed useeless header includes
* Dropped needlessly verbose stuff in _read and _probe functions
* Dropped useless GPL notice
* Lowered log level of shdn pin status in probe function, now it's
  dev_dbg
* Added proper error checking for the GPIO shutdown pin
* remove now always returns zero (man, I've been wrong about this for
  *years* now...)
* Added regulator disable action, cleanup is now handled via devm
* Drop delay_usecs, use delay.value, delay.unit
* Drop config_of, of_match_ptr call
* Dropped IIO_BUFFER, IIO_TRIGGERED_BUFFER dependencies, set SPI_MASTER
  as dependency, fix indenting.
* DT binding: use correct id, add reg description (looks pretty
  standard), dropped spi-max-frequency, fixed dt_binding_check
  complaints (oops!)

Alexandru Lazar (2):
  dt-bindings: iio: adc: Add MAX1241 bindings
  iio: adc: Add MAX1241 driver

 .../bindings/iio/adc/maxim,max1241.yaml       |  64 +++++
 drivers/iio/adc/Kconfig                       |  10 +
 drivers/iio/adc/Makefile                      |   1 +
 drivers/iio/adc/max1241.c                     | 227 ++++++++++++++++++
 4 files changed, 302 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/iio/adc/maxim,max1241.yaml
 create mode 100644 drivers/iio/adc/max1241.c

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2.20.1