regulator: Add support for TPS6287x

Merge series from Mårten Lindahl <marten.lindahl@axis.com>:

This series adds basic support for TI's TPS62870/TPS62871/TPS62872/
TPS62873 high-frequency single-channel step-down converters with an
I2C interface.

The devices can operate in power save mode for maximum efficiency, or
forced-PWM mode for best transient performance and lowest output
voltage ripple. All chip variants have four output voltage ranges and
the driver changes active range depending on the requested voltage
setting.

There are differences in the electrical characteristics and packaging
between the variants, but the register interfaces are identical.