dt-bindings: Add bindings for Tegra DPAUX pinctrl driver

dt-bindings: Add bindings for Tegra DPAUX pinctrl driver

On Tegra124, Tegra132 and Tegra210 devices the pads used by the Display
Port Auxiliary (DPAUX) channel are multiplexed such that they can also
be used by one of the internal I2C controllers. Note that this is
different from I2C-over-AUX supported by the DPAUX controller. The
register that configures these pads is part of the DPAUX controllers
register set and so a pinctrl driver is being added for the DPAUX device
to share these pads. Add the device-tree binding documentation for the
DPAUX pad controller.

Although there is only one group of pads associated with the DPAUX that
can be multiplexed, the group still needs to be described by the binding.
If the 'groups' property is not present in the binding, then the pads
will not be allocated by the pinctrl core for a client and this would
allow another client to re-configure the same pads that may already be
in-use.

Please note that although the "off" function for the DPAUX pads is not
technically a pin-mux setting but more of a pin-conf setting it is
simpler to expose these as a function so that the user can simply select
either "aux", "i2c" or "off" as the current function/mode.

Update the main DPAUX binding documentation to reference the DPAUX pad
controller binding document and add the 'i2c-bus' subnode. The 'i2c-bus'
subnode is used for populating I2C slaves for the DPAUX device so that
the I2C driver core does not attempt to add the DPAUX pad controller
nodes as I2C slaves.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>