Add definitions of Serial1x

[matsujirushi]

Serial1x can communicate Raspberry Pi UART.

Uart Serial1x(&SERCOM_SERIAL1X, PIN_SERIAL1X_RX, PIN_SERIAL1X_TX, PAD_SERIAL1X_RX, PAD_SERIAL1X_TX);
INTERRUPT_HANDLER_IMPLEMENT_SERIAL1X(Serial1x)

void setup() {
  SerialUSB.begin(115200);
  Serial1x.begin(9600);
}

void loop() {
  while (Serial1x.available() >= 1)
  {
    SerialUSB.write(Serial1x.read());
  }
  while (SerialUSB.available() >= 1)
  {
    Serial1x.write(SerialUSB.read());
  }
}

[matsujirushi]

Review comment:

You replaced Serial1 with sercom4, but sercom4 was previously occupied by i2c1, which can cause other problems.

[LynnL4]

Thanks!