Add support for mp2888 device from Monolithic Power Systems, Inc. (MPS)
vendor. This is a digital, multi-phase, pulse-width modulation
controller.

This device supports:
- One power rail.
- Programmable Multi-Phase up to 10 Phases.
- PWM-VID Interface
- One pages 0 for telemetry.
- Programmable pins for PMBus Address.
- Built-In EEPROM to Store Custom Configurations.
- 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>
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/20210511055619.118104-3-vadimp@nvidia.com
[groeck: Add MODULE_IMPORT_NS]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>