regulator: ltc3589: Get rid of struct ltc3589_regulator

After converting to use simplified DT parsing, the struct ltc3589_regulator
only has a desc member. So let's remove struct ltc3589_regulator and use
struct regulator_desc instead.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>