First round of new IIO device support, features and cleanups for the 4.6 cycle.

Device Support
* ad5761
  - new driver
* at91_sama5d2 ADC.
  - new driver and MAINTAINERS entry.
  - minor cleanups followed.
* atlas pH-SM
  - new driver (this has possibly the prettiest data sheet I've ever seen)
* mcp3422
  - mcp3425 ADC added.
* mcp4725
  - mcp4726 DAC added.
* mma8452
  - mma8451q accelerometer added.
* mpl115
  - mpl115a1 added (a lot bigger than it seems as this is an SPI part whereas
    previous parts were i2c).
* si7005
  - Hoperf th02 (seems to be a repackaged part)
* si7020
  - Hoperf th06 (seems to be a repackaged part)

New features
* Core
  - IIO_PH type. Does what it says on the tin.
* max30100
  - LED current configuration support.
* mcp320x
  - more differential measurement combinations.
* mma8452
  - free fall deteciton
- opt3001
  - enable operation without a IRQ line.
  - device tree docs.  Somehow the original docs have disappeared down
    a rabbit hole, so here is a new set.
* st-sensors
  - Support active-low interrupts.

Cleanups and minor / not so minor reworks
* Documentation
  - drop some defunct ABI from the docs in staging.
* presure / Kconfig
  - white space cleanup.
* ad7150
  - BIT macro usage
  - Alignment fixes
* ad7192
  - false indent fixed.
* ak8975
  - constify the ak_def structures
* axp288
  - drop a redundant double const.
* dht11
  - substantial reliability improvements by being more tolerant
    of missing start bits.
  - simplify the decoding algorithm
* mma8452
  - whitespace cleanup
* mpl115
  - don't bother setting i2c_client_data as nothing uses it.
* mpu6050
  - drop unused function parameter.
* opt3001
  - extract integration time as constants.
  - trivial refactoring.
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