iio: mlx90614: Implement filter configuration

Implemented Low pass 3db frequency filter which configures
FIR and IIR values within the configuration register of EEPROM.
For more standardized interface we have fixed the FIR value
to 1024, while changes in IIR value are directly connected to
filter responses. The new datasheet version will provide a
simplified table (also in reStructured text format below) with
this change, to provide quick overview of possible settings.

Below sensor timings (bandwidth) are calculated for 3db frequency
low pass filter.

+--------------------+-----------------+
| Filter setting (%) | Band width (Hz) |
|  (rounded to 1.0)  |                 |
+====================+=================+
|         13         |      0.15       |
+--------------------+-----------------+
|         17         |      0.20       |
+--------------------+-----------------+
|         25         |      0.31       |
+--------------------+-----------------+
|         50         |      0.77       |
+--------------------+-----------------+
|         57         |      0.86       |
+--------------------+-----------------+
|         67         |      1.10       |
+--------------------+-----------------+
|         80         |      1.53       |
+--------------------+-----------------+
|        100         |      7.23       |
+--------------------+-----------------+

The diff is made towards togreg branch. Added myself to MAINTAINERS and
authors as per discussion with Jonathan.

Signed-off-by: Crt Mori <cmo@melexis.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>