All chips of this series with published datasheets support IIN, PIN, and
STATUS_INPUT PMBus commands. Per TI Power Management Forum, "TPS53679 and
TPS53681 have the same PMBus command set". There is no reason to believe
that this does not apply to TPS53688. Let's assume that this is correct
and add support for IIN, PIN, and STATUS_INPUT to TPS53679 and TPS53688
to simplify adding support for more chips of the same series.

At the same time, drop reporting VIN on channel 2. On chips with published
datasheets this voltage is identical to the voltage reported on channel 1,
and there is no reason to believe that this is different for TPS53679 and
TPS53888.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>