iio: magn: add a driver for AK8974

This adds a driver for the Asahi Kasei AK8974 and its sibling
AMI305 magnetometers. It was deployed on scale in 2009 on a
multitude of devices. It is distincly different from AK8973
and AK8975 and needs its own driver.

This patch is based on the long lost work of Samu Onkalo at Nokia,
who made a misc character device driver for the Maemo/MeeGo Nokia
devices, before the time of the IIO subsystem. It was mounted in e.g.
the Nokia N950, N8, N86, N97 etc. It is also mounted on the
ST-Ericsson HREF reference designs.

It works nicely in sysfs:

$ cat in_magn_x_raw && cat in_magn_y_raw && cat in_magn_z_raw
-55
-101
161

And with buffered reads using a simple HRTimer trigger:
$ generic_buffer -c10 -a -n ak8974 -t foo
iio device number being used is 3
iio trigger number being used is 2
No channels are enabled, enabling all channels
Enabling: in_magn_x_en
Enabling: in_magn_y_en
Enabling: in_magn_z_en
Enabling: in_timestamp_en
/sys/bus/iio/devices/iio:device3 foo
-58.000000 -102.000000 157.000000 946684970985321044
-60.000000 -98.000000 159.000000 946684971012237548
-60.000000 -106.000000 163.000000 946684971032257080
-62.000000 -94.000000 169.000000 946684971052185058
-58.000000 -98.000000 163.000000 946684971072204589
-54.000000 -100.000000 163.000000 946684971092224121
-53.000000 -103.000000 164.000000 946684971112731933
-50.000000 -102.000000 165.000000 946684971132232666
-61.000000 -101.000000 164.000000 946684971152191162
-57.000000 -99.000000 168.000000 946684971172210693
Disabling: in_magn_x_en
Disabling: in_magn_y_en
Disabling: in_magn_z_en
Disabling: in_timestamp_en

I cannot currently scale these raw values to gauss. This is
because of lack of documentation. I have sent a request for
a datasheet to Asahi Kasei.

The driver can optionally use a DRDY line IRQ to capture data,
else it will sleep and poll.

Cc: Samu Onkalo <samu.onkalo@intel.com>
Cc: Sebastian Reichel <sre@kernel.org>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Tested-By: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>