dt-bindings: hwmon: add TI DC-DC converters

dt-bindings: hwmon: add TI DC-DC converters

Few Texas Instruments DC-DC converters on PMBus like TPS544B20 do not
have bindings and are used only as hardware monitoring sensor.  These
devices are actually not trivial and can receive basic configuration
(e.g. power up mode, CNTL pin polarity, expected input voltage), however
devicetree support for configuration was never added.

Therefore in current state the devices are used only in read-only mode
and have trivial bindings, so document them to have basic dtschema
tests.

Cc: Vincent Pelletier <plr.vincent@gmail.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20211116110207.68494-1-krzysztof.kozlowski@canonical.com
Signed-off-by: Rob Herring <robh@kernel.org>