freemodbus: fix nvs access failure

place timer handler functions into IRAM
update timer port handlers
fix communication issues
fix offset issue in example
add kconfig option to place handlers into IRAM

components/freemodbus/modbus/include/mbconfig.h

@@ -130,6 +130,9 @@ PR_BEGIN_EXTERN_C
 /*! \brief If the <em>Read/Write Multiple Registers</em> function should be enabled. */
 #define MB_FUNC_READWRITE_HOLDING_ENABLED       (  1 )
 
+/*! \brief Check the option to place handlers into IRAM */
+#define MB_ISR_IN_IRAM                          (  CONFIG_FMB_ISR_IN_IRAM )
+
 /*! @} */
 #ifdef __cplusplus
     PR_END_EXTERN_C