dev/pipe: make pipe a type of device

Pipe have many differences with char device. The main difference is
altough pipe have both read and write, it is unidirectional in nature.

components/drivers/src/pipe.c

@@ -224,7 +224,7 @@ rt_err_t rt_pipe_init(struct rt_pipe_device *pipe,
     pipe->flag = flag;
 
     /* create pipe */
-    pipe->parent.type    = RT_Device_Class_Char;
+    pipe->parent.type    = RT_Device_Class_Pipe;
     pipe->parent.init    = RT_NULL;
     pipe->parent.open    = RT_NULL;
     pipe->parent.close   = RT_NULL;

include/rtdef.h

@@ -739,6 +739,7 @@ enum rt_device_class_type
     RT_Device_Class_SPIDevice,                          /**< SPI device */
     RT_Device_Class_SDIO,                               /**< SDIO bus device */
     RT_Device_Class_PM,                                 /**< PM pseudo device */
+    RT_Device_Class_Pipe,                               /**< Pipe device */
     RT_Device_Class_Miscellaneous,                      /**< Miscellaneous device */
     RT_Device_Class_Unknown                             /**< unknown device */
 };