drm/tegra: dpaux: Add pinctrl support

The DPAUX pins are shared with an internal I2C controller. To allow
these pins to be muxed to the I2C controller, register a pinctrl device
for the DPAUX device.

This is based upon work by Thierry Reding <treding@nvidia.com>.

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