pcnt_unit_enable() function

- ESP_OK: Enable PCNT unit successfully - ESP_ERR_INVALID_ARG: Enable PCNT unit failed because of invalid argument - ESP_ERR_INVALID_STATE: Enable PCNT unit failed because the unit is already enabled - ESP_FAIL: Enable PCNT unit failed because of other error

This function will transit the unit state from init to enable. This function will enable the interrupt service, if it's lazy installed in `pcnt_unit_register_event_callbacks()`. This function will acquire the PM lock if it's lazy installed in `pcnt_unit_set_glitch_filter()`. Enable a PCNT unit doesn't mean to start it. See also `pcnt_unit_start()` for how to start the PCNT counter.