hwmon: (it87) Add support for the IT8721F/IT8758E

Add support for the IT8721F/IT8758E. These new chips differ from the
older IT87xxF chips in the following ways:
* ADC LSB is 12 mV instead of 16 mV.
* PWM values are 8-bit instead of 7-bit.
There are other minor changes we don't have to care about in the
driver.

Another change is that we will handle internal voltage scaling in the
driver instead of delegating the work to user-space.

Signed-off-by: Jean Delvare <khali@linux-fr.org>