Second set of IIO new drivers, cleanups and functionality for the 3.16 cycle.

This set contains a change to the ABI for the hid-sensors drivers to bring them
in line with the long published documentation.  Unfortunately, rather than
reporting true scale and offset values via sysfs they were reporting
some magic numbers that could only be converted to anything useful using
the HID sensors specification. I missed this entirely through the introduction
of a number of drivers, only picking up on it recently.  Srinivas has had
user feedback about this as well. The patch set is too large to go as a fix
at this stage in the cycle and is not a regression fix as this was never
right and so will have to wait for the next merge window. Srinivas assures
me that there are relatively few pieces of hardware out there and he has
had a number of people contact him to point out that the drivers did not
obey the ABI.  Hence hopefully the fallout of this, if any will be minor.
If we don't fix it now, it will only get worse going forward. There is no
sensible way of maintaining the incorrect ABI as it is simply returning
the wrong values through the standard interfaces.

Non IIO elements
* Introduce devm_kmemdup.  Does what it says on the tin.

New drivers:
* hid-sensors rotation devices (output as quaternion)
* Freescale MPL115A2 presure and temperature sensor.
* Melexis mlx90614 contactless infrared sensor.
* Freescale MMA8452Q 3-axis accelerometer.

New functionality:
* Addition of multiple element callback to allow for sysfs interfaces to access
  elements such as quaternions which have no useful meaning if all 4 elements
  are not presented together.  Other future usecases for this include
  rotation matrices.
* Support for multiple element buffer entries for exactly the same uses as
  the sysfs related elements described above.
* Quaternion support via the quaternion IIO modifier.
* TEMP_AMBIENT and TEMP_OBJECT modifiers to distinguish cases with thermopile
  devices.
* hid-sensors gain sysfs access to the sensor readings. Previously these
  drivers used the buffered interface only.  This change involves some
  additional hid-sensors core support to read poll values back from the devices
  to allow the drivers to know roughly how long to wait for a result when
  polling the sensor.  There is also an associated hid-sensors abi to allow
  the devices to be turned off between reads and powered up on demand.

Cleanups and fixes
* Hid sensors fix as described above. Result is to make the _scale and _offset
  attributes applicable in the same way as for all other IIO drivers.
* Some additional documentation - mostly covering stuff that graduated from
  staging without managing to take it's ABI docs with it.
* A series of little tidy ups to the exynos_adc driver that make the code
  nicer to read and improve handling of some corner cases.
* A tidy up to mag3110 (logical fix rather than a real one ;). Also enable
  user offset calibration for this device.
* Drop some left over IS_ERR() checks from ad799x that snuck through during
  the cleanup in the last IIO patch set.
* Fix a naming issue from clashing patches in ak8975 - note the clash only
  occured in the last IIO patch set, hence the fix needs to go through this
  tree.
* A format string missmatch fix in ad7280.c. Unlikely to have ever had an
  impact so not worth rushing through.
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