ARM: tegra: add DT binding for i2c-tegra

Add documentation for device tree binding of NVIDIA's Tegra I2C
controller driver.

Describing all compatible values used for different Tegra SoCs
in details in this documentation.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
[swarren: fixed a couple typos, trimmed examples]
Signed-off-by: Stephen Warren <swarren@nvidia.com>