i2c: atmel: DT binding for i2c driver

DT binding documentation for atmel i2c driver.

Signed-off-by: Songjun Wu <songjun.wu@atmel.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>

diff --git a/doc/device-tree-bindings/i2c/i2c-at91.txt b/doc/device-tree-bindings/i2c/i2c-at91.txt
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+I2C for Atmel platforms
+
+Required properties :
+- compatible : Must be "atmel,at91rm9200-i2c", "atmel,at91sam9261-i2c",
+     "atmel,at91sam9260-i2c", "atmel,at91sam9g20-i2c", "atmel,at91sam9g10-i2c",
+     "atmel,at91sam9x5-i2c", "atmel,sama5d4-i2c" or "atmel,sama5d2-i2c".
+- reg: physical base address of the controller and length of memory mapped
+     region.
+- #address-cells = <1>;
+- #size-cells = <0>;
+- clocks: phandles to input clocks.
+
+Optional properties:
+- clock-frequency: Desired I2C bus frequency in Hz, default value is 100000.
+- Child nodes conforming to i2c bus binding.
+
+Examples :
+
+i2c0: i2c@f8028000 {
+       compatible = "atmel,sama5d2-i2c";
+       reg = <0xf8028000 0x100>;
+       #address-cells = <1>;
+       #size-cells = <0>;
+       clocks = <&twi0_clk>;
+       clock-frequency = <100000>;
+};