iio: adc: Relocate Capacitance to Digital Converters (CDC) into own subdir

No functional changes.
Fix Kconfig description.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>