iio: AK09911 : 3 axis compass support

iio: AK09911 : 3 axis compass support

Added IIO magnetometer driver for AK09911. In functionality is resembles
AK8975 or AK8963. But there are several differences, so instead of
modifying existing AK8975 driver and keep it clean, implemented as
a separate driver.
The key differences are:
- Register map is different and have different indexes
- AK09911 is a very compact interface with no DRDY pin. So no support
 of interrupt or GPIO poll
- Even for polled mode no mention on ST2 register, which is required in
ak8975 driver
- mode values are different for fuse access

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>