iio:magnetometer:ak8975: fix and enhancements

Various fixes and enhancements for the ak8975 magnetometers family driver.

Changes since v1:
* get rid of silly le16 to cpu conversion since SMBUS handles this for us when reading words (patch 3/6)
* get rid of axes caching and serialize bus access between trigger handler and read_raw
* pack triggered buffer sampling bus accesses into a single SMBUS data block access
* use channel address within read_raw

Changes since v2:
* use devm_regulator_get to allow dummy regulator usage
* ensure regulator is properly disabled
* drop first 3 patches since applied
* document in_magn_matrix sysfs attribute
* single line comments where appropriate
* remove explicit zero init for shift field of iio_chan_spec structure as that's the default
* drop unnecessary ak8975_read_axis signature modification
* remove useless cosmetic changes related to ak8975_channels
* normalize and make error handling code paths clearer
* make ak8975_remove less nitpick'able :)

Gregor.

Gregor Boirie (3):
iio:magnetometer:ak8975: fix missing regulator_disable iio:magnetometer:ak8975: mounting matrix support iio:magnetometer:ak8975: triggered buffer support

 Documentation/ABI/testing/sysfs-bus-iio-ak8975     |  46 ++++
 .../bindings/iio/magnetometer/ak8975.txt           |  10 +
 drivers/iio/magnetometer/Kconfig                   |   2 +
 drivers/iio/magnetometer/ak8975.c                  | 255 +++++++++++++++++----
 4 files changed, 271 insertions(+), 42 deletions(-)
 create mode 100644 Documentation/ABI/testing/sysfs-bus-iio-ak8975

2.1.4