iio-sensors: Refactor sensor_catalog definition

iio-sensors: Refactor sensor_catalog definition

Next patches will introduce event support to IIO sensor catalog.
Following the current approach that uses DECLARE_SENSOR_X for
a sensor with X channels, we will have to introduce a macro like
DECLARE_SENSOR_X_Y for a sensor with X channels, each channel having
Y events. But this isn't scalable!

So, instead of using macros for sensor catalog entries lets keep it
simple and use inline definition for each channel. The same will be
done for events.

Change-Id: I12ac7ba07a3115ae85f7ebab835a73fd738bace2
Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>