Add hwmon driver for the Microchip TC74.

The TC74 is a single-input 8-bit I2C temperature sensor,
with +-2 degrees centigrade accuracy.
Signed-off-by: NMaciej Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>