soc/tegra: pmc: Add I/O pad voltage support

I/O pins on Tegra SoCs are grouped into so-called I/O pads. Each such
pad can be used to control the common voltage signal level and power
state of the pins in the given pad.

I/O pads can be powered down even if the system is active, which can
save power from that I/O interface. For SoC generations prior to
Tegra124 the I/O pad voltage is automatically detected and hence the
system software doesn't need to configure it. However, starting with
Tegra210 the detection logic has been removed, so explicit control of
the I/O pad voltage by system software is required.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>