dt-bindings: i2c: Aspeed: Add AST2600 compatible

Document the AST2600 I2C bus compatible string.
Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

Documentation/devicetree/bindings/i2c/i2c-aspeed.txt

-Device tree configuration for the I2C busses on the AST24XX and AST25XX SoCs.
+Device tree configuration for the I2C busses on the AST24XX, AST25XX, and AST26XX SoCs.
 
 Required Properties:
 - #address-cells	: should be 1
@@ -6,6 +6,7 @@ Required Properties:
 - reg			: address offset and range of bus
 - compatible		: should be "aspeed,ast2400-i2c-bus"
 			  or "aspeed,ast2500-i2c-bus"
+			  or "aspeed,ast2600-i2c-bus"
 - clocks		: root clock of bus, should reference the APB
 			  clock in the second cell
 - resets		: phandle to reset controller with the reset number in