[radio-spinel] parse frame rx timestamp (#3731)

This commit parses rx timestamp of radio frame received by RCP, no matter
data frames or ACK frames.

* add OT_NUM_ERRORS as number of defined errors.
* allow empty SPINEL_DATATYPE_DATA and SPINEL_DATATYPE_DATA_WLEN when
parsing spinel packets.

include/openthread/error.h

@@ -347,6 +347,11 @@ typedef enum otError
      */
     OT_ERROR_LINK_MARGIN_LOW = 34,
 
+    /**
+     * The number of defined errors.
+     */
+    OT_NUM_ERRORS,
+
     /**
      * Generic error (should not use).
      */