hwmon: (lm90) Add support for ADT7421

ADT7421 is similar to ADT7461A but supports configurable Beta Compensation.
Packet Error Checking (PEC) is supported but undocumented.

A devicetree node is not added for the added chip since it is quite
unlikely that such an old chip will ever be used in a devicetree based
system. It can be added later if needed.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>