dt-bindings: i2c: nvidia,tegra20-i2c: Add Tegra256 I2C compatible

Add compatible for Tegra256 I2C controllers. Tegra256 consists of
8 generic Tegra I2C controllers similar to previous generations.
The parent clock frequency is different in these controllers and
hence the timing parameter values are different from the previous
ones.

Signed-off-by: Akhil R <akhilrajeev@nvidia.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>