stm32/modmachine: Make RTC class available in machine module.

This is a start to make a more consistent machine.RTC class across ports.
The stm32 pyb.RTC class at least has the datetime() method which behaves
the same as esp8266 and esp32, and with this patch the ntptime.py script
now works with stm32.

ports/stm32/modmachine.c

@@ -384,8 +384,8 @@ STATIC const mp_rom_map_elem_t machine_module_globals_table[] = {
     { MP_ROM_QSTR(MP_QSTR_Pin),                 MP_ROM_PTR(&pin_type) },
     { MP_ROM_QSTR(MP_QSTR_Signal),              MP_ROM_PTR(&machine_signal_type) },
 
-#if 0
     { MP_ROM_QSTR(MP_QSTR_RTC),                 MP_ROM_PTR(&pyb_rtc_type) },
+#if 0
     { MP_ROM_QSTR(MP_QSTR_ADC),                 MP_ROM_PTR(&pyb_adc_type) },
 #endif
 #if MICROPY_PY_MACHINE_I2C