hwmon: (pmbus) Add support for MPS Multi-phase mp2975 controller

Add support for mp295 device from Monolithic Power Systems, Inc. (MPS)
vendor. This is a dual-loop, digital, multi-phase controller.
This device:
- Supports two power rail.
- Provides 8 pulse-width modulations (PWMs), and can be configured up
  to 8-phase operation for rail 1 and up to 4-phase operation for rail
  2.
- Supports two pages 0 and 1 for telemetry and also pages 2 and 3 for
  configuration.
- Can configured VOUT readout in direct or VID format and allows
  setting of different formats on rails 1 and 2. For VID the following
  protocols are available: VR13 mode with 5-mV DAC; VR13 mode with
  10-mV DAC, IMVP9 mode with 5-mV DAC.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Link: https://lore.kernel.org/r/20200926204957.10268-2-vadimp@nvidia.com


[groeck: Cleaned up a couple of error returns; fixed up API changes]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>