esp32/machine_adc: Make ADC 2 optional.

Signed-off-by: IhorNehrutsa <IhorNehrutsa@gmail.com>

ports/esp32/machine_adc.c

@@ -148,11 +148,13 @@ static void mp_machine_adc_print(const mp_print_t *print, mp_obj_t self_in, mp_p
 }
 
 static void madc_atten_helper(const machine_adc_obj_t *self, mp_int_t atten) {
-    esp_err_t err;
+    esp_err_t err = ESP_FAIL;
     if (self->block->unit_id == ADC_UNIT_1) {
         err = adc1_config_channel_atten(self->channel_id, atten);
     } else {
+        #if SOC_ADC_PERIPH_NUM >= 2
         err = adc2_config_channel_atten(self->channel_id, atten);
+        #endif
     }
     if (err != ESP_OK) {
         mp_raise_ValueError(MP_ERROR_TEXT("invalid atten"));