regulator: tps6586x: add support for input supply

There is multiple voltage input pins on device which
takes the voltage input for different voltage regulator.
Support to configure the voltage input supplied by
different regulator for each regulators.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>