ARM: dt: tegra: seaboard: instantiate pinctrl-based I2C bus mux

Tegra's I2C2 controller can be routed to either the PTA or DDC pin group
on Seaboard. Define the pinctrl state nodes required to allow runtime
control of this routing. Instantiate a pinctrl-i2cmux I2C bus mux that
uses these pinctrl states.

Signed-off-by: Stephen Warren <swarren@nvidia.com>