i2c: dt: describe generic bindings

Start a new file which describes the generic bindings used for I2C with
device tree. So we have a central place to look for them, increase
visibility of them, and hopefully reduce the amount of custom properties
introduced.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>