saiport: add support for link training ability query (#1434)

Description
Add support for link-training ability query to suppress redundant link-training requests those are doomed to fail

Motivation and Context
link-training could either be unavailable on certain ports of a switch silicon or totally unsupported on an entire switch silicon
In order to prevent swss and syncd error out from redundant link-training requests, it's better to introduce this port attribute

Signed-off-by: Dante Su <dante.su@broadcom.com>

inc/saiport.h

@@ -2096,6 +2096,14 @@ typedef enum _sai_port_attr_t
      */
     SAI_PORT_ATTR_PFC_TC_DLR_INTERVAL,
 
+    /**
+     * @brief Query link-training support
+     *
+     * @type bool
+     * @flags READ_ONLY
+     */
+    SAI_PORT_ATTR_SUPPORTED_LINK_TRAINING_MODE,
+
     /**
      * @brief End of attributes
      */