iio: temperature: iqs620at-temp: Add support for V3 silicon

The vendor introduced an updated revision of IQS620A(T) silicon
which is backwards-compatible with previous revisions, the only
exception being the offset used to derive temperature.

Enable this new revision by returning the appropriate offset as
a function of the hardware number provided by the parent MFD.

Signed-off-by: Jeff LaBundy <jeff@labundy.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>