hwmon: (adt7410) Document ADT7420 support

The adt7410 driver supports the ADT7420, but its documentation file
makes no mention of that. Add this refrence, and a brief a description
of the differences between the ADT7410 and the ADT7420.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>