hwmon: (pmbus/tps53679) Add support for TPS53647 and TPS53667

TPS53647 and TPS53667 are single channel, Step-Down Buck Controllers.
TPS53647 supports 4 phases, TPS53667 supports 6 phases.
The chips do not support per-phase output telemetry.

Cc: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>