hwmon: (ltc2978) Update datasheet links

Links to datasheets are no longer valid. Update to current links.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
hwmon: (ltc2978) Add support for LTC2974 and LTC3883

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