soc/tegra: pmc: Refactor IO pad voltage control

For Tegra devices, only a subset of IO pads can be configured for 1.8V
or 3.3V. Therefore, in the 'tegra_io_pad_soc' structure for Tegra SoCs
either all or most of the 'voltage' entries are set to UINT_MAX to
indicate the IO pad voltage cannot be configured. So for the majority of
IO pads this configuration is not applicable. However, refactoring the
IO pad data to move this parameter into a separate structure does not
make sense because the benefits are marginal.

Support for the Tegra264 IO pads is currently missing and the control
for configuring the voltage for the IO pads for Tegra264 has changed.
Instead of having a single register that is used for setting the IO pad
voltage for all IO pads, there is now a register associated with the
specific IO pad. For Tegra264, there is now only one IO pad that can be
configured for 1.8V or 3.3V which is the sdmmc1-hv. While we could make
this work with by adding a new SoC flag, the implementation will be a
bit cumbersome. Therefore, it now seems reasonable to refactor the IO
pad code. Hence, introduce a new 'tegra_io_pad_vctrl' structure that
contains the register offset and bit for enabling/disabling 3.3V mode
and move the existing voltage control data for supported SoCs to this
structure. This has an added benefit of simplifying the code in the
functions tegra_io_pad_get_voltage and tegra_io_pad_set_voltage.

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

Author: jonhunter