hwmon: (adt7475) Add support for the ADT7490

Add support for the Analog Devices ADT7490 chip. This chip is largely
compatible with the ADT7473 and ADT7475, with additional features.
In particular, it has 6 voltage inputs instead of 2.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Jordan Crouse <jordan@cosmicpenguin.net>
Cc: "Darrick J. Wong" <djwong@us.ibm.com>