hwmon: (adc128d818) Support operation modes 1-3

Add support for operation modes 1-3 of the ADC128D818 (see datasheet sec.
8.4.1). These differ in the number and type of the available input signals,
requiring the driver to selectively hide sysfs nodes according to the
operation mode configured via devicetree.

Signed-off-by: Alexander Koch <mail@alexanderkoch.net>
Acked-by: Michael Hornung <mhornung.linux@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>