regulator: tps6586x: correct vin pin for sm0/sm1/sm2

As per datasheet, the vin pin for the regulator is named
as vin_sm0, vin_sm1, vin_sm2 for sm0, sm1 and sm2 respectively.

Correcting the names in driver and documentation to match with
datasheet.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reported-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>