Third set of IIO new device support, features and cleanups for the 4.6 cycle.

Good to see several new contributors in this set - and more generally a
number of new 'faces' over this whole cycle.

Staging movements
* hmc5843
  - out of staging.
* periodic RTC trigger
  - driver dropped.  This is an ancient driver (brings back some memories ;)
  that was always somewhat of a bodge. Originally there was a driver that
  never went into mainline that supported large numbers of periodict timers
  on the PXA270 via this route. Discussions to have a generic periodic
  timer subsystem never went anywhere.  At the time RTC periodic
  interrupts were real - now they are emulated using high resolution
  timers so with the HRT driver this has become pointless.

New device support
* mpu6050 driver
  - Add support for the mpu6500.
* TI tpl0102 potentiometer
  - new driver.
* Vybrid SoC DAC
  - new driver.  The ADC on this SoC has been supported for a while, this
    adds a separate driver for the DAC.

New Features
* hmc5844
  - Attributes to configure the bias current (typically part of a self test)
    This could be done before via a somewhat obscure custom interface.
    This at least makes it easy to tell what is going on.
  - Document all custom attributes.
* mpu6050
  - Add support for calibration offset control and readback.
* ms5611
  - power regulator support.  This is always one that gets added the
    first time someone has a board that needs it.  Here it was needed,
    hence it was added.

Cleanups / minor fixes
* tree wide
  - clean up all the myriad different return values in response to a
    failure of i2c_check_functionality.  After discussions everyone seemed
    happy wiht -EOPNOTSUPP which seems to describe the situation well.
    I encouraged a tree wide cleanup to set a good example in future for
    this.
* core
  - Typos in the iio_event_spec documentation in iio.h
* afe4403
  - select REGMAP_SPI to avoid dependency issues
  - mark suspend/resume as __maybe_unused to avoid warnings
* afe4404
  - mark suspend/resume as __maybe_unused to avoid warnings
* atlas-ph-sensor
  - switch the regmap cache type from linear to rbtree to gain reading of
    registers on initial startup.  It's not immediately obvious, but
    regmap flat is meant for high performances cases so doesn't read these
    registers.
  - use regmap_bulk_read in one case where it was using
    i2c_smbus_read_i2c_block_data directly (unlike everything else that was
    through regmap).
* ina2xx
  - stype cleanups (lots of them!)
* isl29018
  - Get the struct device back from regmap rather than storing another
    copy of it in the private data.  This cleanup makes sense in a number
    of other drivers so patches may well follow.
* mpu6050
  - style cleanups (lots of them!)
  - improved return value handling
  - use usleep_range to avoid the usual issues with very short msleeps.
  - add some missing documentation.
* ms5611
  - use the probed device name for the device rather than the driver name.
  - select IIO_BUFFER to avoid dependency issues
* palmas
  - drop IRQF_EARLY_RESUME as no longer needed after genirq changes.
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