drivers: sensor: tmp116: Added ability to read configuration register

Added ability to read contents of TMP116 configuration register by adding
an attribute get function that supports the SENSOR_ATTR_CONFIGURATION
option. Followed the same approach as the TMP108 driver. Interpretation of
the returned value is left up to the caller.

Tested using Nucleo-F401RE and Temp-Log 2 (TMP116) click board and
confirmed a read of the configuration register returned the expected
value.

Signed-off-by: Ian Morris <ian.d.morris@outlook.com>