stm32/main: Call mp_deinit() at end of main.

This adds a call to mp_deinit() in the main function of the STM32 port.
This enables the use of MICROPY_PORT_DEINIT_FUNC on that port, as well as
cleaning up the GIL if threading is enabled.

ports/stm32/main.c

@@ -662,6 +662,7 @@ soft_reset_exit:
     MICROPY_BOARD_END_SOFT_RESET(&state);
 
     gc_sweep_all();
+    mp_deinit();
 
     goto soft_reset;
 }