drivers: hwmon: (pmbus/ltc2978) Add support for LTM4686 uModule

This patch adds support for LTM4686 Ultrathin Dual 10A or
Single 20A uModule Regulator with Digital Power System Management.

Datasheet: http://www.analog.com/ltm4686



Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>