hwmon: (pmbus/tps53679) Extend device list supported by driver

Extends driver with support of the additional devices:
Texas Instruments Dual channel DCAP+ multiphase controllers: TPS53688.

Extend Kconfig with added device.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Link: https://lore.kernel.org/r/20200113150841.17670-4-vadimp@mellanox.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>