regulator: ltc3589: Fix module dependency

Make this driver depend on I2C and select REGMAP_I2C to fix build failure.
Also allows this driver to be built as module.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>