regulator: dt-bindings: Add bindings for TPS6287x

Add bindings for the TPS62870/TPS62871/TPS62872/TPS62873 voltage
regulators.

Signed-off-by: Mårten Lindahl <marten.lindahl@axis.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230502-tps6287x-driver-v3-1-e25140a023f5@axis.com


Signed-off-by: Mark Brown <broonie@kernel.org>