core: pinmux: implement rough solution

[facchinm]

While we prepare a cleaner solution, this patch allows back and forth between pinmuxes.
Tested on UNO Q with next patch in the series and this sketch

void setup() {}
void loop() {
  Serial.begin(115200);
  Serial.println("helloooooo");
  delay(20);
  Serial.end();
  delay(20);
  pinMode(1, OUTPUT);
  digitalWrite(1, HIGH);
  delay(10);
  digitalWrite(1, LOW);
  delay(100);
  analogWrite(1, 33);
}

Pin 1 correctly prints the string, muxes as GPIO and then produces the required PWM