regulator: tps65218: Add support for LS2

Re-use the "tps65218_pmic_*_current_limit()" functions of LS3
and calculate the different required bit-shift by counting the
trailing 0s in "struct regulator_desc.csel_mask"

Signed-off-by: Christian Hohnstaedt <Christian.Hohnstaedt@wago.com>
Signed-off-by: Mark Brown <broonie@kernel.org>