hwmon: (pmbus/isl68137) Add driver for Intersil ISL68137 PWM Controller

Intersil ISL68137 is a digital output 7-phase configurable PWM
controller with an AVSBus interface.

Signed-off-by: Maxim Sloyko <maxims@google.com>
Signed-off-by: Robert Lippert <rlippert@google.com>
Signed-off-by: Patrick Venture <venture@google.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>