regulator: ltc3589: Use maple tree register cache

regulator: ltc3589: Use maple tree register cache

The ltc3589 can only support single register read and write operations
so does not benefit from block writes. This means it gets no benefit from
using the rbtree register cache over the maple tree register cache so
convert it to use maple trees instead, it is more modern.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230609-regulator-ltc-maple-v1-1-08c15181f8b2@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>