regulator: tps65218: add support for LS3 current regulator

Add support for TPS65218 LS3 current regulator, which is capable of 4
current input limit modes: 100, 200, 500, and 1000 uA.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Signed-off-by: Mark Brown <broonie@kernel.org>