hwmon: (lm70) Add TI TMP121 support

The Texas Instruments TMP121 is a SPI temperature sensor very similar
to the LM70, with slightly higher resolution.  This patch extends the
LM70 driver to support the TMP121.  The TMP123 differs in pin assign-
ment.

Signed-off-by: Manuel Lauss <mano@roarinelk.homelinux.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>