dt-bindings: Add MP2888 voltage regulator device

Monolithic Power Systems, Inc. (MPS) dual-loop, digital, multi-phase
controller.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210511055619.118104-4-vadimp@nvidia.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>