regulator: tps6586x: add SMx slew rate setting

Add output vlotage slew rate setting for SM0/SM1

Signed-off-by: Xin Xie <xxie@nvidia.com>
Signed-off-by: Danny Huang <dahuang@nvidia.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>