add more device definition.

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1457 bbd45198-f89e-11dd-88c7-29a3b14d5316

include/rtdef.h

@@ -567,6 +567,9 @@ enum rt_device_class_type
 	RT_Device_Class_RTC,							/**< RTC device 								*/
 	RT_Device_Class_Sound,							/**< Sound device 								*/
 	RT_Device_Class_Graphic,						/**< Graphic device                             */
+	RT_Device_Class_I2C, 							/**< I2C device                                 */
+	RT_Device_Class_USBDevice,						/**< USB slave device                           */
+	RT_Device_Class_USBHost,						/**< USB host bus								*/
 	RT_Device_Class_Unknown							/**< unknown device 							*/
 };