hwmon: (adc128d818) Implement mode selection via dt

Implement operation mode selection using the optional 'ti,mode' devicetree
property (see [1]). The ADC128D818 supports four operation modes differing
in the number and type of input readings (see datasheet, sec. 8.4.1), of
which mode 0 is the default.

We only add handling of the 'ti,mode' property here, the driver still
supports nothing else than the default mode 0.

[1] Documentation/devicetree/bindings/hwmon/adc128d818.txt

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>