mcpwm_timer_register_event_callbacks() function

- ESP_OK: Set event callbacks successfully - ESP_ERR_INVALID_ARG: Set event callbacks failed because of invalid argument - ESP_ERR_INVALID_STATE: Set event callbacks failed because timer is not in init state - ESP_FAIL: Set event callbacks failed because of other error

The first call to this function needs to be before the call to `mcpwm_timer_enable` User can deregister a previously registered callback by calling this function and setting the callback member in the `cbs` structure to NULL.