include: rtc: move ds3231 doc to rtc doxygen group

Fold the documentation of this header under RTC docs

Signed-off-by: Benjamin Cabé <[email protected]>

Author: kartben

include/zephyr/drivers/rtc/maxim_ds3231.h

@@ -209,7 +209,7 @@ extern "C" {
 /**
  * @brief RTC DS3231 Driver-Specific API
  * @defgroup rtc_ds3231_interface RTC DS3231 Interface
- * @ingroup io_interfaces
+ * @ingroup rtc_interface
  * @{
  */