hwmon: (ltc2978) Add support for LTC3886

LTC3886 is a is a dual PolyPhase DC/DC synchronous step-down switching
regulator controller. It is mostly command compatible to LTC3883,
but supports two phases instead of one.

Suggested-by: Michael Jones <mike@proclivis.com>
Tested-by: Michael Jones <mike@proclivis.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>