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>

Documentation/devicetree/bindings/i2c/i2c.txt

+Generic device tree bindings for I2C busses
+===========================================
+
+This document describes generic bindings which can be used to describe I2C
+busses in a device tree.
+
+Required properties
+-------------------
+
+- #address-cells  - should be <1>. Read more about addresses below.
+- #size-cells     - should be <0>.
+- compatible      - name of I2C bus controller following generic names
+		    recommended practice.
+
+For other required properties e.g. to describe register sets, interrupts,
+clocks, etc. check the binding documentation of the specific driver.
+
+The cells properties above define that an address of children of an I2C bus
+are described by a single value. This is usually a 7 bit address. However,
+flags can be attached to the address. I2C_TEN_BIT_ADDRESS is used to mark a 10
+bit address. It is needed to avoid the ambiguity between e.g. a 7 bit address
+of 0x50 and a 10 bit address of 0x050 which, in theory, can be on the same bus.
+Another flag is I2C_OWN_SLAVE_ADDRESS to mark addresses on which we listen to
+be devices ourselves.
+
+Optional properties
+-------------------
+
+These properties may not be supported by all drivers. However, if a driver
+wants to support one of the below features, it should adapt the bindings below.
+
+- clock-frequency	- frequency of bus clock in Hz
+- wakeup-source		- device can be used as a wakeup source.