soc/tegra: pmc: Implement tegra_io_pad_is_powered()

Implement a function to query whether a pad is in deep power down mode.
This is needed by the pinctrl callbacks.

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