hwmon: (pmbus) add support for 2nd Gen Renesas digital multiphase

Extend the isl68137 driver to provide support for 2nd generation Renesas
digital multiphase voltage regulators.

Signed-off-by: Grant Peltier <grantpeltier93@gmail.com>
Link: https://lore.kernel.org/r/62c000adf0108aeb65d3f275f28eb26b690384aa.1584720563.git.grantpeltier93@gmail.com
[groeck: Adjusted for new PMBus API function parameters]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>