Add support for Texas Instruments TPS6287x, single-channel
synchronous step-down converters with four output voltage ranges.

Signed-off-by: Mårten Lindahl <marten.lindahl@axis.com
Link: https://lore.kernel.org/r/20230502-tps6287x-driver-v3-2-e25140a023f5@axis.com
Signed-off-by: Mark Brown <broonie@kernel.org