samples: sensor: fdc2x1x: Add FDC2X1X sample application

This sample application periodically reads raw data from the FDC2X1X
sensor in polling mode or optionally with data ready trigger.
It is able to read the 12-Bit and 28-Bit, as well as the 2-Channel
and 4-Channel versions (FDC2112, FDC2114, FDC2212, FDC2214).
The 4-channel versions are chosen through devicetree properties.
The default is FDC2112.

Signed-off-by: Igor Knippenberg <igor.knippenberg@gmail.com>