soc/tegra: pmc: Implement pad configuration via pinctrl

Register a pinctrl device and implement get and set functions for
PIN_CONFIG_LOW_POWER_MODE and PIN_CONFIG_POWER_SOURCE parameters.

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