soc/tegra: pmc: Remove public pad voltage APIs

Make tegra_io_pad_set_voltage() and tegra_io_pad_get_voltage() static
and remove the prototypes from pmc.h. Remove enum tegra_io_pad_voltage
and use the defines from <dt-bindings/pinctrl/pinctrl-tegra-io-pad.h>
instead.

These functions aren't used outside of the pmc driver and new use cases
should use the pinctrl interface instead.

Signed-off-by: Aapo Vienamo <avienamo@nvidia.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>